What is a day? You might casually talk about a day as a period of daylight. Or you could measure a day in relationship to the sun or the stars. Astronomers use the term solar day to describe a day relative to the sun. A solar day is the time from one solar noon – one local noon or high noon – to the next. It’s the interval between successive days as marked by the sun’s highest point in our sky. If you look at a day in that way, you can say that the longest days of the year come each year around the December solstice … no matter where you live on the globe.

Available now! 2024 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year! And it makes a great gift.

The longest days are in December

What? Isn’t the shortest day for the Northern Hemisphere at the December solstice? Yes, it is, if we are talking about the period of daylight.

But, we’re talking about the (approximately) 24-hour interval from one solar noon to the next. In December, a day – one rotation of Earth relative to the noonday sun – is about half a minute longer than the average 24 hours, for the entire globe.

Keep in mind that the clocks on our walls don’t measure the true length of a day, as measured from solar noon to solar noon. To measure that sort of day, you’d need a sundial. A sundial will tell you the precise moment of local solar noon, when the sun reaches its highest point in the sky each day.

Days are always longer – as measured from one solar noon to the next – than 24 hours around the solstices, and less than 24 hours around the equinoxes.

Why are the days longer in December?

The days are at their longest now – for the entire globe – because we’re closer to the sun on the December solstice than we are at the June solstice. Earth’s perihelion – closest point to the sun – always comes in early January. So when we’re closest to the sun, our planet is moving a little faster than average in its orbit. That means our planet is traveling through space a little farther than average each day.

The result is that Earth has to rotate a little more on its axis for the sun to return to its noontime position. That effect lengthens the solar day by about eight seconds. In contrast, at aphelion, when the Earth is moving slower in its orbit, the solar day is about seven seconds shorter.

There’s another effect that happens during both the winter and summer solstices that increases the solar day by 21 seconds. It’s due to the way the sun moves mostly eastward, in relation to the stars, during solstices. Therefore, when the sun rises and moves up in the sky, it takes a bit longer to reach high noon from the previous day’s high noon.

For the winter solstice, the combined effects of these two phenomena increase the solar day by about 29 seconds.

Half a minute longer doesn’t sound like much, but the difference adds up. For instance, two weeks before the December solstice, noontime comes about seven minutes earlier by the clock than on the December solstice. And then two weeks after the December solstice, noon comes about seven minutes later by the clock than on the December solstice itself.

Sunrises and sunsets

Because the clock and sun are most out of sync right now, some befuddling phenomena cause people to scratch their heads at this time of the year. In the Northern Hemisphere, the year’s earliest sunsets precede the December winter solstice. And the year’s latest sunrises come after the December winter solstice. So the earliest sunsets came earlier in December for most of us; and the latest sunrises won’t come until early January.

In the Southern Hemisphere, the year’s earliest sunrises precede the December summer solstice, and the year’s latest sunsets come after the December summer solstice.

The fact that we’re closest to the sun in early January also means that Northern Hemisphere winter (Southern Hemisphere summer) is the shortest of the four seasons. Read more about the shortest season here.

However, at the same time … It’s the season of bountifully long solar days.

Visit Sunrise Sunset Calendars to find out the clock time for solar noon at your locality; remember to check the Solar noon box.

Bottom line: As measured from one solar noon to the next, December has the longest days – the longest interval from the sun’s highest point on one day to its highest point on the next day – for the entire Earth. And that’s true, no matter where you live on the globe.

A planisphere is virtually indispensable for beginning stargazers. Order your EarthSky Planisphere today!

The post Longest days accompany the December solstice first appeared on EarthSky.

Earliest sunset isn’t on the shortest day

Have you noticed your sunsets coming later now? That’s true for many of us, even though the December solstice is still more than a week away. For the mid-northern U.S. and similar latitudes – around 40 degrees north latitude – the earliest sunsets of the year came around December 8. That would be the latitude of New York City; Philadelphia, Pennsylvania; Kansas City, Missouri; Denver, Colorado; Reno, Nevada; Beijing, China; Madrid, Spain; and Naples, Italy.

For more southerly latitudes – say around 30 degrees north latitude – the earliest sunsets of the year came in late November and early December.

And what about the Southern Hemisphere? At this same time, the year’s earliest sunrises have happened or are happening, as you progress toward your longest day at the December solstice.

Then, closer to the Arctic and Antarctic Circles, the earliest sunset and earliest sunrise happen nearer the solstice.

Want to know the date of your earliest sunset (or sunrise)? Sunrise-sunset.org provides the sunrise/sunset times to the second for locations around the globe.

The exact date of the Northern Hemisphere’s earliest sunset and the Southern Hemisphere’s earliest sunrise varies by latitude.

But at temperate latitudes, both of these annual hallmarks in our sky come a few to several weeks before the December solstice, not at the solstice as you might expect.

Available now! 2024 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year! And it makes a great gift.

Why?

The next solstice in 2023 comes at 3:27 UTC on December 22 and marks an unofficial beginning for winter in the Northern Hemisphere. So for the Northern Hemisphere, this upcoming solstice brings the shortest day and longest night of the year. Then why isn’t the earliest sunset on the year’s shortest day?

Basically, it’s because of the discrepancy between the clock and the sun. A clock ticks off exactly 24 hours from one noon to the next. But an actual day – as measured by the spin of the Earth, from what is called one solar noon to the next – rarely equals 24 hours exactly.

Also, solar noon is simply called midday, because it refers to that instant when the sun reaches its highest point for the day. Thus, in the month of December, the time period from one solar noon to the next is actually half a minute longer than 24 hours. For example, in Philadelphia, Pennsylvania on December 7 the sun reaches its noontime position at 11:52 a.m. local standard time. Then, two weeks later – on the winter solstice – the sun will reach its noontime position around 11:58 a.m. So that’s six minutes later than on December 7.

Visit Sunrise Sunset Calendars to know the clock time for sunrise, solar noon and sunset plus day length in your part of the world, remembering to check the solar noon and day length boxes.

Another key point is that the later clock time for solar noon means a later clock time for sunrise and sunset. This can be seen in the table below.

For Philadelphia, Pennsylvania

Latest sunrise and earliest sunset aren’t on the solstice

As you might have guessed, the latest sunrises aren’t on the day of the solstice either. For middle latitudes in the Northern Hemisphere, the latest sunrises come in early January.

Although there’s variation in the exact dates, the sequence is always the same for both hemispheres. First, earliest sunset before the winter solstice, the winter solstice itself, latest sunrise after the winter solstice. Then, half a year later, earliest sunrise before the summer solstice, the summer solstice itself, latest sunset.

So by all means, check out the earliest and latest sunsets and sunrises in your area. They are always lovely and happen around every solstice.

Sunsets from the EarthSky Community

Bottom line: The solstice comes on December 22, 2023, at 3:27 UTC. Does that coincide with the earliest sunsets? It depends on where you live. The earliest sunsets at mid-northern latitudes happen weeks before the solstice. By comparison, latitudes closer to the equator have their earliest sunsets in late November, or earlier in December. And then latitudes closer to the Arctic Circle will have their earliest sunsets closer to the December solstice.

Solstice tale of two cities: New York, New York, and St. Augustine, Florida

EarthSky’s monthly night sky guide: Visible planets and more

The post The earliest sunset comes before the winter solstice first appeared on EarthSky.

Menkar in Cetus the Whale

Menkar isn’t the brightest star in Cetus the Whale. Instead, Diphda – aka Deneb Kaitos, the Whale’s Tail – is brightest. And Menkar isn’t even the Whale’s most famous star. That honor goes to Mira, sometimes called Mira the Wonderful, a renowned variable star.

Still, Menkar is an interesting star, with its own claims to fame. It’s the 2nd-brightest star in Cetus at a +2.6 magnitude. And it’s designated the alpha star in Cetus. Why? It could be because Menkar sits closer to the ecliptic, which marks the sun’s yearly circuit in front of the background stars.

What’s more, Star Trek fans might recognize the name Menkar. Keep reading to learn more about this star.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

The Whale’s alpha star in TV and film

Hello, Star Trek fans! Remember the star that played a key role in an original Star Trek episode called Space Seed (1967)? And the second Star Trek film Star Trek II: The Wrath of Khan (1982) also featured it.

Khan and his crew are exiled to the planet Ceti Alpha V in Space Seed, and they escape from there in Wrath of Khan after the planet becomes uninhabitable. Ceti Alpha. Alpha Ceti. It’s named after Menkar, despite its flipped Bayer designation.

By the way, as of December 5, 2023, there are 5,552 confirmed exoplanets in 4,091 planetary systems, with 887 systems having more than one planet. However, there are no known planets around Menkar (Alpha Ceti).

Star Trek II: The Wrath Of Khan (1982) was the first film to have a scene created entirely by CG graphics and the first time a movie had been released that was a sequel to a TV show episode, following on as it does from the Star Trek episode Space Seed (1967). pic.twitter.com/FWwhhEZZOK

— FilmFrame (@FilmFrameATRM) August 9, 2019

When is the best time to see Menkar?

As seen from northern temperate and polar latitudes, Menkar shines due south around 9 p.m. your local time around the December winter solstice (on or near December 21). Because all the stars, except the sun, return to the same place in the sky about 2 hours earlier with each passing month, that places Menkar highest up and due south around 7 p.m. on January 21.

The V-shaped Hyades star cluster – Face of the Bull in the constellation Taurus – serves as a directional arrow, pointing the way to the star Menkar. Hold your fist at arm’s length. Then hop two fist-widths from the point of the “V” to locate Menkar’s approximate position in the star-studded heavens.

It’s pretty easy to see Menkar and the circle of stars outlining the Head of Cetus the Whale in a dark sky. In the lore of the sky, this fearsome creature almost gobbled up the Princess Andromeda, the daughter of King Cepheus and Queen Cassiopeia. But Perseus the hero rescued the Ethiopian princess in the nick of time.

Alpha Ceti is a pulsating red giant star ? 249 light years away in Cetus

Notes: About 48 times the radius and 380 times brighter than our Sun ?

(Click link to explore the Milky Way ? and beyond)https://t.co/rOzvZewby5 pic.twitter.com/um1xbmbvLr

— MR Humble ????? (@Todd_Scheve) February 14, 2019

Science of Menkar

A small telescope shows Menkar to be a colorful double star. The orange giant star, Menkar, contrasts with its fainter blue-white companion, the star 93 Ceti. These two stars are not physically related. They just happen to lie along our line of sight, as seen from Earth. Menkar resides some 250 light-years away, while 93 Ceti lies at about twice that distance away.

Menkar’s ruddy complexion reveals that it’s a cool star in the autumn of its years, and 93 Ceti’s blue-white radiance indicates a hot star in the heyday of youth.

Menkar has about twice the sun’s mass. However, Menkar is in a different stage of its evolution than our sun. In fact, it has exhausted the hydrogen and helium fuel at its core and has expanded to become a giant star with about 89 times the radius of our sun. The large area of Menkar’s photosphere – or visible outer layer – means that it is emitting about 1,455 times as much energy as the sun, even though the effective temperature is only 3,795 K (compared to 5,778 K on the sun). The low temperature is the reason Menkar shines with a reddish hue.

Red stars are cooler than white stars, just as red hot on Earth indicates something cooler than white hot.

As Menkar continues to evolve – and to burn carbon at its core – it’s thought this star will become unstable and ultimately become a variable star. So, perhaps it’ll be similar to Mira, Cetus the Whale’s famous variable star.

Bottom line: The star Menkar is the alpha star in the constellation Cetus the Whale. History, lore, claims to fame, and science information here.

Enjoying EarthSky? Sign up for our free daily newsletter today!

The post Menkar – a visual double – is the Whale’s alpha star first appeared on EarthSky.

Mirfak is the brightest star in the constellation Perseus the Hero. This star and its constellation lie high in the east-northeast on December evenings, as seen from the Northern Hemisphere. It makes a great companion to the sky’s most celebrated eclipsing binary star, which is called Algol, sometimes called the Ghoul Star or Demon Star. Mirfak and Algol are about 10 degrees, or a fist-width, apart in Perseus. As you watch the variable star Algol dip and then regain brightness, compare it to the more constant Mirfak. You’ll soon see why Algol perplexed the early stargazers, and why Mirfak seemed steadfast in comparison.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

How to find Mirfak

The name Mirfak comes from Arabic and means the Elbow of the Pleiades. Mirfak is also another name for Alpha Persei. In fact, the constellation Perseus lies due north of the Pleiades star cluster, also called the Seven Sisters. You can find Mirfak and Perseus between the Pleiades cluster and Polaris, the North Star.

Or you can also take a more direct route to Mirfak, if you’re familiar with the M or W-shaped constellation Cassiopeia the Queen. Draw an imaginary line through the Cassiopeia stars Navi (Gamma Cassiopeiae) and Ruchbah to jump over to Mirfak.

Mirfak is the lone star of Perseus that stands out in moderately light-polluted skies, as its brilliance matches that of the stars of the famous Big Dipper.

Science of Alpha Persei

At a distance of about 510 light-years, Mirfak lies much farther than the Big Dipper stars, so this star has to be intrinsically very luminous to shine so brightly in our sky. If Mirfak stood at the sun’s distance from Earth, its disk would cover several thousand times more sky. Moreover, Mirfak would shine thousands of times more brightly than our sun. In fact, it’s about 5,000 times more luminous than our sun.

On a dark night, you might discern a faint array of stars clustering around Mirfak, a bejeweled realm of the heavens that glitters all the more in binoculars. This assemblage of stars is known as the Alpha Persei Moving Group (Melotte 20), of which Mirfak is the most prominent member. Although some feel that this stellar grouping is too dispersed to be called a star cluster, these stars nonetheless move in the same general direction through space and were born from the same cloud of gas and dust some 30 to 50 million years ago.

Bottom line: Mirfak is the brightest star in the constellation Perseus and is visible even from light-polluted locations. This star is a member of the Alpha Persei Moving Cluster of stars.

The post Mirfak is the brightest star in Perseus the Hero first appeared on EarthSky.

Come to know the legendary Pleiades star cluster

The Pleiades star cluster is also famously known as the Seven Sisters. Or, to some, it’s known as Messier 45 (M45) on the list of Messier objects. The Pleiades is visible from almost every part of the globe. It’s seen from as far north as the North Pole and farther south than the southernmost tip of South America. It looks like a tiny misty dipper of stars.

In fact, many ancient cultures had myths and stories associated with the Pleiades. In Greek mythology, the Pleiades were the seven daughters of Atlas, a Titan who held up the sky, and the oceanid Pleione, protectress of sailing. The sisters were Maia, Electra, Alcyone, Taygete, Asterope, Celaeno and Merope. The Pleiades were sometimes said to be nymphs in the train of Artemis. They were half-sisters of the seven Hyades. The Hyades pattern is another star cluster near the Pleiades stars.

According to a Polynesian legend, the Pleiades was once a single star: the brightest in the sky. The Polynesian god Tane disliked this star bragging about its beauty. So the god smashed the star into pieces, creating the Pleiades star cluster.

The modern-day festival of Halloween originates from an old Druid rite that coincided with the midnight culmination of the Pleiades cluster. People believed the veil dividing the living from the dead is at its thinnest when the Pleiades culminates – reaches its highest point in the sky – at midnight.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Sibling stars traveling through space together

In both myth and science, the Pleiades are sibling stars. Modern astronomers say the stars were born from the same cloud of gas and dust some 100 million years ago. This gravitationally bound cluster of several hundred stars looms some 430 light-years distant. Also, these sibling stars drift through space together at about 25 miles per second (40 kilometers per second). Many of these stars shine hundreds of times more brightly than our sun.

How to find the Pleiades

If you’re familiar with the famous constellation Orion the Hunter, it can help you confirm you’ve found the Pleiades. See the three stars in a row in Orion? That’s Orion’s Belt. Now draw a line through these stars to the V-shaped pattern of stars with a bright star in its midst. The V-shaped pattern is the Face of Taurus the Bull. The bright star in the V – called Aldebaran – depicts the Bull’s Eye. Then, when you go a bit past Aldebaran, you’ll see the Pleiades cluster. It marks the Bull’s Shoulder.

In our Northern Hemispheres skies, the Pleiades cluster is associated with the coming winter season. It’s easy to imagine this misty patch of icy-blue suns as hoarfrost clinging to the dome of night. So frosty November is the month of the Pleiades, because it’s at this time that the Pleiades shines from dusk until dawn. But you can see the Pleiades cluster in the evening sky well into April.

The Pleiades and Aldebaran

The star name Aldebaran comes from an Arabic word for follower. It’s thought to be a reference to this star’s forever chasing the Pleiades across the heavens. As a general rule, the Pleiades cluster rises into the eastern sky before Aldebaran rises, and sets in the west before Aldebaran sets.

The only exception to this rule happens at far southern latitudes. For example, at South America’s Tierra del Fuego, Pleiades rises a short while after Aldebaran rises.

Legend of the Lost Sister

Most people see six, not seven, Pleiades stars in a dark country sky.

However, the story about the lost 7th Pleiad appears universal. The astronomer Robert Burnham Jr. found the lost Pleiad myth prevalent in the star lore of European, African, Asian, Indonesian, Native American and Aboriginal Australian populations.

Moreover, Burnham suggested that the “lost Pleiad” may have basis in fact. Modern astronomy has found that the 7th-brightest Pleiades star – Pleione – is a complicated shell star that goes through numerous permutations. Naturally, these changes can cause this star to vary in brightness.

Plus, people with exceptional eyesight have been known to see many more stars in the cluster. Claims go up as high as 20 stars. Agnes Clerke, an astronomer and writer in the late 1800s, reported that Michael Maestlin, the mentor of Johannes Kepler, mapped out 11 Pleiades stars before the invention of the telescope.

To see more than six or seven Pleiades stars, you must have very good eyesight (or a pair of binoculars). And you must be willing to spend time under a dark, moonless sky. Stephen O’Meara, a dark-sky connoisseur, claims that eyes dark-adapted for 30 minutes are six times more sensitive to light than eyes dark-adapted for 15 minutes. But the surest way to see additional Pleiades stars is to look at this cluster using some optical aid.

The Pleiades photo gallery

Bottom line: The Pleiades – or Seven Sisters – is a star cluster that’s a popular target for observers in the late fall and winter in the Northern Hemisphere.

The post The Pleiades – or 7 Sisters – known around the world first appeared on EarthSky.

Look for Mintaka in Orion’s Belt

See the three stars at Orion the Hunter’s midsection? These stars are Alnitak, Alnilam and Mintaka. In fact, they’re very noticeable and famous in many cultures as Orion’s Belt. In the Southern Hemisphere, the Aymara people of Bolivia, Peru and Chile saw the Belt as a celestial bridge or a staircase to the world of souls. But astronomers see the Belt as a type of bridge, too. These stars link the sky’s northern and southern hemispheres.

Consider Mintaka, which is the Belt’s westernmost star. It sits almost directly astride the celestial equator: the projection of Earth’s equator onto the stellar sphere.

So where can you find Mintaka and the constellation Orion? In late November, from around the world, Orion rises in your eastern sky around 9 p.m. and climbs highest for the night around 1 to 2 a.m. local time.

When dawn is breaking, or about to break, say around 5 to 6 a.m., the Hunter sits low in your western sky.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Mintaka shines on the celestial equator

Mintaka’s location on the celestial equator makes it a good guidepost for finding directions here on Earth. That is, Mintaka and the other stars of the Celestial Bridge are visible worldwide. From all over the world, Mintaka rises due east, sets due west, and remains in the sky for 12 hours. It climbs to its highest point in the sky midway between rising and setting.

When it’s highest in the sky, if this star shines at your zenith (your straight-overhead point), then you must be at the equator.

If this star shines in the southern half of your sky, then you must be north of the equator.

If this star shines in the northern half of your sky, then you must be south of the equator.

The story of the Celestial Bridge is one of many about the constellation Orion. That’s because it’s so noticeable on our sky’s dome. So watch for it when you’re outside one evening soon!

Bottom line: The indigenous Aymara people of the Andes and Altiplano regions of South America see the famous sky feature we know as Orion’s Belt as a celestial bridge between the sky’s Northern and Southern Hemispheres. In fact, its westernmost star, Mintaka, lies directly on the celestial equator.

The post See Orion’s Belt as a celestial bridge between hemispheres first appeared on EarthSky.

Book review for “Totality”

Oscar Wilde once quipped that a book exhausting its subject only succeeds in exhausting its readers. I’m proud to present a book that stands as a notable exception to this rule: Totality: The Great North American Eclipse of 2024 by Mark Littmann and Fred Espenak. It’s one of the most enjoyable reads that I’ve had in a long time.

Edifying as well as entertaining, Littmann and Espenak masterfully interweave eclipse history, folklore, superstition, geography, science and technology. “Totality” is sure to charm people across the board, from novice to maven. The friendly and conversational style almost makes one forget to be intimidated by technicalities.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Totality is nature’s grandest visual spectacle

Yes, a whole cavalcade of geeks contributed to the production of this great opus. Even so, these eclipse aficionados are more interested in sharing their joyous passion than in imposing a dissertation upon you. They – as much as anyone – can sympathize with the exasperating words of Alfonso X, King of Castile (1252), who complained:

If God had consulted with me before embarking upon creation, I would have recommended something simpler.

The theme of the book is simple and can’t be repeated often enough: A total eclipse of the sun counts as nature’s grandest visual spectacle. In fact, an eclipse chaser who witnessed 30 total solar eclipses hardly exaggerates in proclaiming:

In rating natural wonders, on a scale of 1 to 10, a total eclipse of the sun is a million.

Total solar eclipse once in 375 years

A total solar eclipse only comes once to your part of the world in an average of 375 years. If you live in North America, and the path of this upcoming total solar eclipse passes through your neighborhood, take full advantage. That’ll probably be your once-in-lifetime opportunity to witness a total eclipse of the sun in your own backyard. Absolutely nothing compares to the ultimate experience of standing inside a moon shadow.

Alas, the average figure of 375 years stated above proves to be quite misleading overall. For instance, people in Carbondale, Illinois, get to see two total solar eclipses in the span of seven years (2017 and 2024). Yet, the next total solar eclipse in Antwerp, Belgium, (on May 25, 2142) will be the first to occur there in at least seven centuries. (According to Jean Meeus in Mathematical Astronomy Morsels I, page 92.) No wonder King Alfonso X admonished the creator for the hard-to-fathom cosmos.

Treasure trove of information

The authors are begging and pleading with all those living on or near the path of the total solar eclipse (and those willing to travel) to enjoy nature’s greatest visible spectacle. Most likely, another total eclipse of the sun won’t happen in your neck of the woods for hundreds of years. So let this treasure trove of information ready you for the thrill of a lifetime.

Our tour guides – Mark Littmann and Fred Espenak – say in way of conclusion:

If the weather cooperates, the total eclipse of the sun on April 8, 2024, could mark the biggest outdoor spectator event in American history – a 2,140-mile-long (3,444-km-long) tailgate party to watch the heavenly performance of the moon and sun.

Bottom line: Read a book review of “Totality: The Great North American Eclipse of 2024” by Mark Littmann and Fred Espenak. Reviewed by Bruce McClure.

The post Media we love: Totality, The Great North American Eclipse of 2024 first appeared on EarthSky.

November 3, 2023, ushers in the earliest solar noon – for the whole year and for the entire globe – by the clock. Solar noon is a natural rather than artificial construct, although our clocks and calendars measure its continual shift throughout the year. Solar noon – aka midday – refers to that passing instant when the sun reaches its highest point for the day, midway between sunrise and sunset. If you reside along the center line of your time zone, then solar noon – or midday – comes at 11:44 a.m. by your clock every year in early November. If you’re not on your time zone’s center line – and most of us aren’t – solar noon happens a bit earlier or later by your clock, depending on your offset from your time zone’s center line.

Obscure? Possibly. But the shift in solar noon is tied to a phenomenon that many people around the world might notice in the coming weeks. That is, the earliest solar noon of the year is a prelude to the year’s earliest sunset in the Northern Hemisphere – and the year’s earliest sunrise in the Southern Hemisphere.

You thought the earliest sunset for the Northern Hemisphere came at the December solstice? It doesn’t. It happens well before that solstice, for a reason that’s related to the earliest solar noon on November 3.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Earliest and latest solar noons, and the length of the day. The first several days of November, from everywhere worldwide, the days (as measured from one solar noon to the next) are almost exactly 24 hours in duration. Thereafter, day by day, solar noon comes later and later by the clock, lessening the discrepancy between sun time and clock time. At long last – on December 25, 2023 – the sun and the clock agree with one another, with solar noon coming at 12 noon by the clock (for those at the center line of a given time zone).

Although the sun and clock will agree on December 25, 2023, the duration of the solar day – as measured from one solar noon to the next – will continue to exceed 24 hours each day for another six weeks.

Finally, the latest solar noon will arrive on or near February 11, at 12:14 p.m. by the clock (at the time zone center line).

And, for everyone, everywhere worldwide, solar noon comes some 30 minutes earlier by the clock on November 3, 2023, than it will on February 11, 2024.

Want to know the clock time for solar noon (midday) at your location? Visit Sunrise Sunset Calendars, remembering to check the solar noon box.

Want to know the time difference between sun and clock for any date? Go to AstroPixels.com and look under the equation of time column.

Solar noon and earliest sunset (and/or sunrise). So … November 3, 2023, marks the earliest solar noon of the year for the whole globe. What’s more, residents at the Earth’s equator have their earliest sunrise and sunset on November 3. That’s because, at the equator, the daylight hours remain virtually the same throughout the year – so the earliest sunrise, earliest solar noon and earliest sunset all fall on the same day.

Elsewhere – within the tropical and temperate regions of the world – the shortest period of daylight happens on the day of the winter solstice, and the longest period of daylight on the summer solstice. So if you live near the Arctic Circle, your earliest sunset doesn’t happen until around the December (winter) solstice; or if you live near the Antarctic Circle, your earliest sunrise doesn’t take place until around the December (summer) solstice.

That’s in spite of the earliest solar noon coming in early November all over the world.

Read more: December solstice starts shortest season

If you live north of the equator but south of the Arctic Circle, your earliest sunset occurs somewhere between November 3, 2023, and December 21, 2023. In the Northern Hemisphere, those living closer to the equator have an earlier date for their earliest sunset; and those lodging closer to the Arctic Circle have a later date for their earliest sunset.

We give the approximate dates for the earliest sunset in the Northern Hemisphere:

15 degrees north latitude
Earliest sunset on or near November 23

30 degrees north latitude
Earliest sunset on or near December 1

45 degrees north latitude
Earliest sunset on or near December 10

60 degrees north latitude
Earliest sunset on or near December 17

In the Southern Hemisphere, it’s your earliest sunrise that takes place between November 3, 2023, and December 21, 2023. The closer you live to the equator, the earlier the date for the earliest sunrise; and the closer you live to the Antarctic Circle, the later the date for the earliest sunrise. Because comparable latitudes north/south of the equator have the same approximate dates for earliest sunset/earliest sunrise, the dates for the Southern Hemisphere’s earliest sunrises can be gleaned from the listing above.

In the Northern Hemisphere, the latest sunrise must wait till after the Northern Hemisphere’s December winter solstice; and in the Southern Hemisphere, it’s the latest sunset that must wait till after the Southern Hemisphere’s December summer solstice. Those living closer to the Arctic or Antarctic Circles have an earlier date for their latest sunrise/latest sunset; and those lodging closer to the Earth’s equator have a later date for their latest sunrise/latest sunset. Here are the approximate dates for the Northern Hemisphere’s latest sunrise/Southern Hemisphere’s latest sunset at various latitudes:

60 degrees latitude: December 27
45 degrees latitude: January 2
30 degrees latitude: January 11
15 degrees latitude: January 23

Two reasons account for the unequal length of the solar day over the year. First and foremost, the tilt of the Earth’s axis causes the solar day to be more than 24 hours long around the solstices yet less than 24 hours long around the equinoxes. However, Earth’s eccentric orbit plays a secondary role, either accentuating or lessening the effect. At the December solstice, the Earth is some 3 million miles (5 million km) closer to the sun, and moving more swiftly in its orbit, than on the June solstice. Therefore, the longer-than-average solar day at the December solstice (24 hours + 30 seconds) outlasts the longer-than-average solar day at the June solstice (24 hours + 13 seconds).

Bottom line: November 3 brings the year’s earliest solar noon – that is, earliest midday – by nature’s clock. It’s a harbinger of the Northern Hemisphere’s earliest sunset.

Need a good sky almanac? Try these recommended almanacs

The post Year’s earliest solar noon on November 3 first appeared on EarthSky.

Halloween is a cross-quarter day

Sure, Halloween is the modern-day descendant of Samhain, a sacred festival of the ancient Celts and Druids in the British Isles. And, yes, Halloween is short for All Hallows’ Eve. But, at its heart, Halloween is an astronomy holiday. It’s a day rooted in Earth’s orbit around the sun. It’s a cross-quarter day and a testament to our ancestors’ deep understanding of the sky.

The cross-quarter days fall more or less midway between the equinoxes (when the sun sets due west) and solstices (when the sun sets at its most northern or southern point on the horizon). Halloween – October 31 – is approximately midway between our Northern Hemisphere autumn (September) equinox and winter (December) solstice In the Southern Hemisphere, the September equinox heralds spring and the December solstice, summer.

In other words, in traditional astronomy, there are eight major seasonal subdivisions of every year. They include the March and September equinoxes, the June and December solstices, and the intervening four cross-quarter days.

In modern times, the four cross-quarter days are often called Groundhog Day (February 2), May Day (May 1), Lammas (August 1) and – the most sinister cross-quarter day because it comes at a dark time of year – Halloween (October 31).

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Halloween falls at a dark time of year

For us in the Northern Hemisphere, Halloween is the darkest of the cross-quarter days. And it comes at a time of year when the days are growing shorter. Early people once said that the spirits of the dead wander from sunset until midnight around this cross-quarter day. After midnight – on November 1, now called All Saints’ Day – the ghosts supposedly go back to rest.

The October 31 date for Halloween is fixed by tradition. The true cross-quarter day falls on November 7, representing a discrepancy of about a week. According to the ancient Celts, a cross-quarter day marks the beginning – not the middle – of a season.

The Pleiades connection

Some believe that the early forebear of Halloween – Samhain – happened on the night that the Pleiades star cluster culminated at midnight.

In other words, the Pleiades climbed to its highest point in the sky at midnight on or near the same date as this cross-quarter day. Now, the midnight culmination of the Pleiades cluster now occurs on November 21, but Halloween is fixed on October 31.

Presuming the supposed connection between Samhain and the midnight culmination of the Pleiades, the two events took place on or near the same date in the 11th century (1001-1100) and 12th century (1101-1200). This was several centuries before the introduction of the Gregorian calendar.

At that time, when the Julian calendar was in use, the cross-quarter day and the midnight culmination of the Pleiades fell – amazingly enough – on or near October 31. But, then, the Julian calendar was about one week out of step with the seasons. Had the Gregorian calendar been in use back then, the date of the cross-quarter day celebration would have been November 7.

Calendar converter via Fourmilab

But Halloween falls on October 31 now. Meanwhile, the true cross-quarter day happens on or near November 7. And the midnight culmination of the Pleiades cluster is on or near November 21.

Pleiades is associated with Halloween

Bottom line: October 31, the date for Halloween, marks the approximate midway point between the September equinox and the December solstice. So Halloween is an astronomy holiday, and one of the year’s four cross-quarter days.

Read about another cross-quarter day, Groundhog Day

May Day is a cross-quarter day

Enjoying EarthSky? Sign up for our free daily newsletter today!

The post Halloween is an astronomy holiday. It’s a cross-quarter day first appeared on EarthSky.

Why is Algol the Demon Star?

What’s the scariest star in the sky? If you were one of the early stargazers, you might have chosen Algol in the constellation Perseus. That is probably why early astronomers nicknamed Algol the Demon Star. Shivers!

When you look at Algol, it doesn’t appear any scarier than any other star. But in skylore it’s associated with a mythical scary monster – the Gorgon Medusa – who had snakes for hair. Legend said that her appearance was so terrifying that if anyone even looked at her, they would turn to stone.

The star Algol takes its name from an Arabic word meaning the Demon’s Head or, literally the Ghoul. It represents the terrifying snaky head of the Medusa monster.

Perseus was a great hero often depicted mounted on Pegasus the Flying Horse. In the mythology of the skies, Perseus slew Medusa. Then, he used Medusa’s head to his advantage, showing it to Cetus the Sea-monster to turn him into stone. So, perhaps the ancients associated this star’s variable brightness with the evil, winking eye of the Medusa.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

Algol is a variable star

Winking? Yes. In fact, Algol is a known variable star, which waxes and wanes in brightness.

The early stargazers surely knew about its changing brightness. This probably led them to name the strangely behaving star in the sky for a mythological demon.

There are many variable stars known throughout the heavens, but Algol might well be the most famous of them all. The Demon Star brightens and dims with clockwork regularity. It completes one cycle in 2 days, 20 hours and 49 minutes. Plus, you can view its entire cycle with your eye alone.

Algol’s variation is easy to observe. At its brightest, Algol shines about three times more brightly than at its faintest. When it reaches maximum brilliance, Algol matches the brightness of the nearby second-magnitude star Almach. At minimum, Algol’s light output fades to that of the star Epsilon Persei.

Modern-day astronomy has unlocked the secret of Algol’s mood swings. It’s an eclipsing binary star. This kind of binary star is composed of two stars, with each star revolving around the other. From Earth, we see the orbital plane of this binary star almost exactly edge-on. Therefore, when the dimmer of the two stars swings in front of the brighter star, we see Algol at minimum brightness.

How to find Algol the Demon Star

The Demon Star is easy to find. Our sky chart shows the northeastern sky for autumn evenings, especially around Halloween.

The conspicuous W or M-shaped constellation Cassiopeia enables you to star-hop to Perseus. Look below Cassiopeia toward the horizon to spot the dangling icicle shape of Perseus. Off to the right of the icicle is Algol. At mid-northern latitudes, the Demon Star appears for at least part of the night all year round. But it’s best seen in the evening sky from autumn to spring. It’s visible in the northeast sky in autumn, shines high overhead in winter, then swings to the northwest sky by spring.

Bottom line: Algol has the nickname the Demon Star because it represents the head of Medusa. This variable star probably intrigued the ancients with its fluctuating behavior.

The post Algol the Demon Star of Perseus first appeared on EarthSky.