On Tuesday, October 10, 2023, you can see a lovely conjunction in the morning sky. The dazzling planet Venus and bright star Regulus – Heart of the Lion in Leo – will meet on the sky’s dome. By coincidence, the waning crescent moon will be visible that morning also! What a glorious sight in the east before dawn!

Technically speaking, objects are said to be in conjunction when they have the same right ascension – sort of like celestial longitude – on our sky’s dome. Practically speaking, objects in conjunction will likely be visible near each other for some days. So be sure to watch for Venus and Regulus on the days before and after the October 10 conjunction, too.

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What is a conjunction?

Occasionally, two or more objects meet up with each other in our sky. Astronomers use the word conjunction to describe these meetings. The word conjunction comes from Latin, meaning to join together. Maybe you remember the old Conjunction Junction cartoons from the 1970s. In language, conjunctions relate to clauses brought together with words like and. In astronomy, conjunctions relate to two or more objects brought together in the sky.

Sometimes one of these objects is the sun, so the conjunction cannot be seen. But other conjunctions – between stars, our moon, and the planets – can be truly spectacular.

You can’t see an inferior conjunction

An inferior conjunction is when an object passes between us and the sun. Any object that orbits the sun closer than Earth does might pass through inferior conjunction from time to time. That is, it can, assuming its orbit lies more or less close to the ecliptic.

Usually, though, when astronomers speak of an inferior conjunction, they’re talking about Venus or Mercury, which orbit between Earth and the sun. Astronomers sometimes refer to Venus and Mercury as inferior planets. When they’re at or near inferior conjunction, we generally can’t see them. They’re hidden in the sun’s glare. Occasionally, though, Venus or Mercury at inferior conjunction can be seen to transit across the sun’s disk.

We shouldn’t forget the moon here. It passes between Earth and the sun at new moon once each month. Therefore it would be correct, if a little unusual, to say that the moon is at inferior conjunction when it’s at its new phase.

You can’t see a superior conjunction either

A superior conjunction is when an object passes behind the sun from our point of view. Look at Venus’ orbit in the diagram above. Half of its conjunctions with the sun – when they come together on our sky’s dome – are inferior conjunctions, and half are superior conjunctions. It’s fun to imagine the inferior planets on an endless cycle of passing in front of the sun, as seen from Earth, then behind it, and back again, like squirrels running around a tree.

Meanwhile, the superior planets – or planets farther from the sun than Earth – can never be at inferior conjunction. Mars, Jupiter, Saturn, Uranus and Neptune can never pass between us and the sun. So the superior planets only have superior conjunctions.

But other conjunctions can look beautiful

The most common – and most exciting – type of conjunction doesn’t involve the sun. Any time two objects pass each other on the sky’s dome, they’re said to be at conjunction. These sorts of conjunctions – maybe between two planets, or a planet and a star, or a star and the moon – happen multiple times every month. They are beautiful. The view can stop you in your tracks.

For example, if you were fortunate enough to have looked at the moon on July 21, 1969, as Neil Armstrong and Buzz Aldrin headed home from the Sea of Tranquillity, you’d have seen the moon in conjunction with Spica, the brightest star in the constellation Virgo. They were only about two degrees apart that night. That’s a bit more than the width of your index finger held out at arm’s length.

There are always a few particularly good conjunctions every year. On March 1-2, 2023, we were treated to a spectacular conjunction between bright planets Venus and Jupiter, as you can see below. Click here to see a full gallery of Venus-Jupiter conjunction photos captured by members of the EarthSky community.

Watch for and enjoy conjunctions

People often think about the night sky as being permanent and unchanging, at least on a human scale. If you watch the skies often, though, you’ve surely noticed that’s not true. The stars don’t move relative to each other, but they do move across the sky over the course of a single night, as Earth spins under the sky. And, from one night to the next, each star rises and sets four minutes earlier each day, as Earth moves around the sun.

Once you’ve found the ecliptic, you can see where the real action is. Because they are relatively close to us, the planets and moon do move relative to each other and the stars, and quickly, from our point of view. They change their positions, appear to move closer together and farther apart, and sometimes pass by each other in the sky coming to conjunction. Of all of the pleasures of stargazing, seeing this movement of our nearest neighbors is one of the greatest.

Stay up to date with upcoming conjunctions via EarthSky’s night sky guide.

Bottom line: A conjunction is when two objects are close together on our sky’s dome. Some conjunctions can’t be seen, but others look spectacular.

The post A conjunction happens when 2 worlds meet first appeared on EarthSky.

You might have heard that opposition is the best time of year to observe a planet. But what is opposition? And which planets have oppositions? In astronomy, opposition means a planet is opposite the sun. So, for example, the planets with orbits inside Earth’s orbit (Mercury and Venus) can’t be in opposition. But the planets orbiting outside Earth’s orbit (Mars, Jupiter, Saturn, Uranus, Neptune) all can.

Once each year, we pass between them and the sun in our smaller, faster orbit. If the sun is setting in the west, and Jupiter is rising in the east, then Jupiter is at opposition. Earth is passing between the sun and Jupiter, which will, therefore, take all night to cross our sky. At opposition, a planet is easiest to observe because it’s generally closest to Earth and visible throughout the night.

Look at the diagram above. The sun is in the center of the diagram. Earth is a little way out, and Saturn farther still. As seen from above our solar system, the planets are moving counterclockwise around the sun. Now, run this diagram forward in your mind. Because it’s in an inner orbit, Earth travels faster than Saturn. And Jupiter (not shown) travels faster than Saturn. Of course, the planets will be in various configurations as seen from above the solar system. But, in nearly every earthly year, Earth will pass between the outer planets and the sun. This is opposition.

An opposition as seen from Earth

At opposition, Earth is in the middle of a line between an outer planet and the sun, and we see the sun at one end of our sky and the opposition planet in the opposite direction. It’s as if you’re standing directly between two friends as you chat in the supermarket, and you need to turn your head halfway around to see one and then the other. At opposition, the sun is on the opposite side of the sky from the outer planet; when the sun sets in the west, the planet is rising in the east. As the planet drops below the horizon, the sun pops above it again: opposite.

To be technical, opposition for an outer planet happens when the sun and that planet are exactly 180 degrees apart in the sky. The word comes to English from a Latin root, meaning to set against.

Consider that Venus and Mercury can never be at opposition as seen from Earth. Their orbits are closer to the sun than Earth’s, so they can never appear opposite the sun in our sky. You will never see Venus in the east, for example, when the sun is setting in the west. These inner planets always stay near the sun from our point of view, no more than 47 degrees from the sun for Venus, or 28 degrees for Mercury.

Oppositions can only happen for objects that are farther from the sun than Earth is. We see oppositions for Jupiter, Saturn, Uranus and Neptune about every year. They happen as Earth, in its much-faster orbit, passes between these outer worlds and the sun. We see oppositions of the planet Mars, too, but Martian oppositions happen about every 27 months, because Earth and Mars are so relatively close together in orbit around the sun. Their orbits, and speeds in orbit, are more similar.

Dates of upcoming oppositions

Since everything in space is always moving, oppositions of planets farther than us from the sun happen again and again. As far as the bright planets go, the next opposition is never too far away:

Mars was at opposition on December 8, 2022, and will be again on January 15, 2025.

Jupiter was at opposition on September 26, 2022, and will be again on November 2-3, 2023.

Saturn was at opposition on August 14, 2022, and will be again on August 26-27, 2023.

Uranus will be at opposition on November 9, 2022, and next on November 13, 2023.

Neptune will be at opposition on September 16, 2022, and next on September 19, 2023.

Note that Jupiter’s oppositions come about a month later each year. Saturn’s come about two weeks later each year. Uranus and Neptune opposition dates come only a few days later, year after year.

Why are planets at opposition so interesting to sky-watchers?

As mentioned, because they’re opposite the sun, planets at opposition rise when the sun sets and can be found somewhere in the sky throughout the night.

Secondly, planets at opposition tend to be near their closest point to Earth in orbit. Due to the non-circular shape of planetary orbits, the exact closest point might be different by a day or two, as will be the case for Jupiter in 2023. Jupiter’s opposition is on November 2-3, 2023, and its exact closest point will be on November 1-2. Still, for many weeks around opposition – between the time we pass between an outer planet and the sun – the outer planet is generally closest to Earth. At such a time, the planet is brightest, and more detail can be seen through telescopes.

And here’s another interesting aspect of opposition. Since the sun and outer planet are directly opposite each other in Earth’s sky, we see that far-off planet’s fully lit daytime side. Fully lit planets appear brighter to us than ones not completely lit. If you’re saying to yourself that this sounds a lot like the moon, you’re right! After all, what’s a full moon if not the moon at opposition? During the moon’s full phase, it’s directly opposite the sun in the sky, fully illuminated, and at its brightest for that orbit. As it moves through the rest of its orbit, the sun-Earth-moon line bends and gives us what we see from Earth as the moon’s phases.

Oppositions from other planets

Like so much in life, opposition is all about point of view. We’ve been talking about the view from Earth. What if we flip it around?

When an outer planet – let’s say Jupiter – is at opposition for us, Earth is at inferior conjunction as seen from that planet. In other words, at the moment of opposition for us on Earth, observers on Jupiter would see Earth passing between their world and the sun. The Earth and the sun would be in the same side of Jupiter’s sky, Earth hidden in the sun’s glare except to skilled observers using special equipment.

Consider also that the line from the sun to Jupiter passes through the Earth, which means Earth passes directly between the sun and Jupiter. Maybe one day, a visitor to Jupiter will see Earth transit the sun as seen from Jupiter. That is, they’ll see Earth’s darkened nighttime side, and all of humanity, cross the face of the sun from half a billion miles away.

Bottom line: The ideal time to observe a planet is around opposition. During opposition, Earth passes between an outer planet and the sun, placing the planet opposite the sun in our sky. A planet at opposition is closest to Earth in its orbit and bright. It rises when the sun sets, and so is visible all night.

Read more: Saturn at opposition August 26-27, 2023

Read more: Jupiter at opposition November 2-3, 2023

The post What does opposition mean for an outer planet? first appeared on EarthSky.

The ecliptic is the sun’s path

Maybe you’ve noticed that the moon and planets follow the sun’s path across your sky? Unless you live in the high Arctic or Antarctic, you see the sun arc across your sky each day from east to west. The moon follows the sun’s path. And so do the major planets in our solar system. This imaginary track across our sky is the ecliptic. Technically speaking, it’s a projection of the plane of Earth’s orbit around the sun, traced onto our sky. Practically speaking, the ecliptic forms a great circle around the sky and is a useful tool for stargazers.

Why do the moon and planets follow the sun’s path? It’s mainly because, long ago – before there was a solar system as we know it today – there was a vast cloud of gas and dust in space. This cloud was spinning, and, as it spun, it flattened out. Our sun formed in the center of this cloud. The major planets (including Earth) and most other solar system objects formed in the flat disk surrounding the sun. The ecliptic is this flat disk of planets in our sun’s family – our solar system – translated onto our sky.

Planets follow the ecliptic

So the major planets – and many of the minor planets, aka asteroids – orbit the sun in more or less the same plane. We can speak of this plane as defined by Earth’s orbit around the sun: the ecliptic.

If we could watch the solar system from far above the Earth’s North Pole, we’d see the planets, moons, asteroids and some of the comets (but not all of them) rushing around the sun counterclockwise in this plane, like marbles rolling around a flat dish. In fact, the major planets are more within the dish than on it. They’re within the plane of the ecliptic, more or less.

They retain the outline of the original cloud in space from which they were born, and their movement around the sun is an echo of the original spin of the cloud.

Defining the zodiac

Far beyond the cold edges of our solar system, we see the stars of our Milky Way galaxy. And the stars are moving, too, in great orbits around the galaxy’s center. But they’re so far away that they don’t seem to move over the course of a human lifespan. That’s why we refer to them as “fixed” stars.

Naturally, the fixed stars on the ecliptic – or sun’s path – seemed special to the early stargazers. So they identified constellations made of these stars, and used the word zodiac for the wider pathway traveled by these constellations. And so we find the sun, moon and our major planets within the constellations of the zodiac.

A clarification on the ecliptic

Now about that phrase we keep using, the phrase more or less …

The fact is, the other planets don’t orbit exactly in the Earth-sun plane. That’s because each major planet’s orbit is inclined a little bit to this plane. Some of the asteroids have orbits that are more inclined. And comets tend to have the most inclined orbits of all. See a chart at Wikipedia with the inclinations of the major planets’ orbits.

The moon and the ecliptic

Interestingly, Earth’s moon isn’t exactly on the ecliptic, either. Its orbit around Earth is tilted by about 5.15 degrees relative to the ecliptic. This means the moon spends most of its time above or below the ecliptic. It crosses it twice each orbit: once going upward and once downward from our point of view. Therefore, we usually see the moon close to, but not exactly alongside, the other solar system objects.

On the other hand, the moon sometimes passes right in front of other solar system objects, in an event called an occultation.

So there are little variations. But – for all practical purposes of skywatching – you can think of the ecliptic as a line across our sky. You can think of the sun, moon and major planets of the solar system as moving along that line.

The ecliptic and the season

One thing to remember, though. The sun’s path is high in summer and low in winter. So the location of the ecliptic in your sky shifts a bit, seasonally. Also, the angle of the ecliptic and how it connects to our horizon is different in spring and fall and in the evening versus the morning. See the charts below.

Ecliptic, eclipse

If the word ecliptic sounds familiar, you’re right. That’s because it’s from the same root as the word eclipse, from the Latin and Greek meaning to “fail to appear” or “to be hidden.” Of course, the moon hides the sun during an eclipse. So the ecliptic got its name because the ancients saw that solar eclipses happen when the moon crosses the ecliptic during the new moon phase.

Later, astronomers gave the name node to the places where the moon crosses the ecliptic. If the moon traveled exactly on the ecliptic, and the other planets did, too, the moon would occult, or block out, all the planets and the sun every orbit. We’d have lunar and solar eclipses every month!

Watch the sky

If you’re able, keep an eye on the sun, the moon and the planets for a while. Watch for a few days, a few weeks, months, years, even. You’ll begin to get a feel for the ecliptic in your sky. You’ll notice the planets, sun and moon are always on or near the ecliptic, and you can use this line across your sky to help you find your way around, making your way between the constellations and stars. You’ll notice the sun’s path – the ecliptic – higher in the sky during the summer months and lower during the winter.

Eventually, you’ll be able to imagine the sun’s path in your sky, long after the sun has set.

When that happens, you’ll be able to pick out a planet from a star very quickly and easily, which is a great party trick. Mars is the red one; Saturn the yellow one; Venus the bright white one that never gets too far from the sun; Mercury the seldom-seen one; and Jupiter the very bright one (but never as bright as Venus) that often gets far from the sun.

Welcome to stargazing, friend!

Bottom Line: The ecliptic is the path the sun takes across our sky. In fact, it’s the Earth-sun plane. And, more or less, it’s the plane of the orbits of the major planets and their moons, and some asteroids. Here’s a stargazing tip: Learn the whereabouts of the ecliptic in your sky. And you’ll always find the sun, moon and planets on or near it.

The post The ecliptic is the sun’s path in our sky first appeared on EarthSky.

The Spring Triangle heralds warmer weather

Around the time of the March equinox, a trio of wide-spread stars rises in the east after dark. The Spring Triangle announces the slide into shorter nights and warmer weather. Regulus in Leo the Lion is the first to rise above the horizon, having risen before the sun has even set. It’s followed by Arcturus in Boötes the Herdsman. And, just a bit later, Spica in Virgo the Maiden joins the group. These three bright stars create a narrow pyramid stretching up from the horizon.

The Spring Triangle is entirely above the horizon before midnight in March. And by early April, its three stars are visible by mid-evening (midway between sundown and midnight).

Once you come to know it, when you see the Spring Triangle stars above the houses across the street, you can almost feel the warm springtime air.

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The Spring Triangle is an asterism

Like the sky’s other seasonal shapes (for instance, the Summer Triangle and Winter Circle or Hexagon), the Spring Triangle isn’t a constellation. It’s not one of the 88 regions of the sky officially recognized as constellations by the International Astronomical Union.

Instead, it’s an asterism, an unofficial but recognizable pattern of stars that can be in one constellation or in multiple constellations. Asterisms are what many of us would pick out as constellations, if we didn’t know any constellations. That’s because they’re often the sky’s most recognizable patterns.

Let’s look at how to find these stars so we can watch them move across the night sky.

Regulus

As soon as it’s dark in March, look for a bright yellowish star twinkling above the eastern horizon. That’s Regulus, and it’s easy to confirm if you’ve spotted the right star. If the star you’re targeting marks the period in a backward question mark pattern of stars, you’ve got it. This question mark shape is another asterism known as the Sickle in Leo. The curve of the question mark traces the head of the lion and Regulus is the Lion’s heart.

When we look at Regulus we only see one star, but it’s actually a four-star system. From about 79 light-years away, the light from the four stars makes one point of light in the night sky. The brightest star in this system is a yellow supergiant about three times the size of our sun.

Arcturus

Next up is Arcturus, the brightest star of the three in the Spring Triangle. For those at northerly latitudes, Arcturus is the second-brightest star visible on the sky’s dome, after Sirius, which is currently in the southwestern sky. (Those at more southerly latitudes, like the southern U.S., can see the sky’s actual second-brightest star, Canopus, in the south.) Arcturus is a gorgeous old red giant about 37 light-years away. Billions of years in the future, when the sun has burnt up its own hydrogen fuel supply, it will turn into a star similar to the type Arcturus is now.

Spica

If Arcturus has risen, Spica is not far behind. Look for Spica lower in the sky than Arcturus – and father toward the south, or right – of the others. From a distance of 262 light-years away, Spica appears to us on Earth to be a lone bluish-white star in a quiet region of the sky. But Spica consists of two stars and maybe more. The pair are both larger and hotter than our sun, and they’re separated by only 11 million miles (less than 18 million km). They orbit their common center of gravity in only four days.

A triangle inside the triangle

If you can spot the Spring Triangle, you may notice there’s a second triangle inside the larger triangle. The smaller triangle excludes Regulus but includes yellowish Denebola, a double star about 36 light-years away that marks the Lion’s tail. Denebola is the second brightest in Leo. To see this second triangle, see the chart below.

The Spring Triangle is less attention-grabbing than the Winter Circle (or Hexagon) and the Summer Triangle. If you’re having trouble finding it, there’s another way. Use the Big Dipper for extra help.

Finding the Spring Triangle

Toward the north, look for the Big Dipper, called the Plough in the United Kingdom. This time of year, by mid-evening, it’s ascending in the northeast. If you draw a line from the two stars at the end of the Dipper’s bowl or blade – Dubhe and Merak – and extend it toward the south, you’ll reach Regulus.

Then, follow the curve of the Dipper’s handle away from the bowl to arc to Arcturus and continue the line downward to speed on down to Spica.

Surprisingly enough, the Spring Triangle is bigger than its more famous summertime cousin, and it’s almost as big across as the Winter Hexagon. Yet it’s not one of the best-known star patterns.

Once you’ve found the Spring Triangle, you’ll enjoy it year after year. Maybe because it appears as spring is about to arrive, this pattern seems full of optimism for good things to come!

Bottom Line: Look for a sign of the changing seasons in the heavens as the Spring Triangle, made up of the stars Regulus, Arcturus and Spica, rises above the horizon in the east over the next couple of months.

Read more: Arc to Arcturus, the springtime star

The post Spring Triangle rises late at night, heralding the season first appeared on EarthSky.