September 2020 guide to the bright planets

Wow! Fiery red Mars – now nearly at its best for 2 years – will be near the Harvest Moon in late September and early October. Jupiter and Saturn are very noticeable, too, bright and near each other. Mercury is up after sunset, especially for Southern Hemisphere viewers. Venus blazes in the east at dawn.

Click the name of a planet to learn more about its visibility in September 2020: Jupiter, Saturn, Mars, Venus, Mercury

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The moon goes by the planets Jupiter and Saturn in the September 2020 evening sky.

Look for the waxing gibbous moon near the planets Jupiter and Saturn for several days centered around September 24 and 25, 2020. Read more.

Harvest Moon and the red planet Mars at nightfall close to line of ecliptic.

In late September and early October 2020, the Northern Hemisphere’s Harvest Moon shines in the vicinity of the brilliant red planet Mars! Read more.

Steep line of ecliptic with Mercury and Spica near horizon.

This chart is for the Southern Hemisphere, where people will enjoy the best evening apparition of Mercury for the year in September 2020. As seen from the Northern Hemisphere, Mercury will be deeply submerged in evening twilight and hard to see. Read more.

Mars is now rising over your eastern horizon not long after the sun goes down, heading for its once-in-two-years opposition on October 13, 2020. In other words, we’ll be passing between the sun and Mars then. At this wondrous time, with Mars so close, it is actually supplanting Jupiter as the sky’s fourth-brightest celestial body, after the sun, moon, and the planet Venus. It’s very bright and very red now and altogether something to see!

Beginning around mid-September, as seen from around the globe, Mars is ascending in the east in early evening to mid-evening. You can’t miss it. No other object in our evening sky (with the exception of the moon) is so bright, and certainly none is so red in color.

By around the last evening of September – and in the first few evenings of October – Mars will be near the nearly full or entirely full Harvest Moon. Big bright Mars next to a big bright Harvest Moon? What could be better?

Read more: Everything you need to know about 2020’s Harvest Moon

As the weeks go by, Mars will be rising earlier. By mid-October, it’ll be rising in the east as the sun sets in the west.

After that, for the remainder of this year, Mars will be in our sky at sunset, fading in brightness as the year draws to a close, but still … a sight to see.

Chart showing Mars rising in the east as Saturn and Jupiter appear in the south.

2 other planets – Jupiter and Saturn – are up at the same time as Mars. A line between them points to Mars. From the Northern Hemisphere, Jupiter and Saturn are in the south as Mars is ascending in the east. You can’t miss all 3 planets, no matter where you are on Earth, but their orientation with respect to your horizon will shift, depending on where you are. For your exact view, try Stellarium.

Jupiter and Saturn are near one another on the sky’s dome throughout September, with Saturn following Jupiter westward across the sky from nightfall until well after midnight. In July 2020, these gas giant worlds reached their yearly oppositions, when Earth – in its yearly orbit – swung between these outer worlds and the sun. Thus we were closest to Jupiter and Saturn for the year in July. Jupiter and Saturn, in turn, were at their brightest and out all night. Now they’re in the sky for fewer hours of the night, but still dazzlingly noticeable and very beautiful. They are headed for a great conjunction in December 2020.

Read more: Jupiter at opposition on July 13-14

Read more: Saturn at opposition on July 20

Sky chart: Jupiter and Saturn on line of ecliptic, and the Teapot asterism.

If you notice just one object in the sky after sunset, it might be very bright Jupiter. This planet outshines all the stars, plus it’s near another bright planet, Saturn. You can’t miss these two. Jupiter and Saturn climb highest up for the night at early evening in early September, and at nightfall by the month’s end. Read more. Before 2020 ends, Jupiter and Saturn will undergo a great conjunction.

In some respects, though, September gives us a better month than July or August for viewing Jupiter and Saturn. That’s because these two worlds remain bright and beautiful throughout September, yet appear highest up for the night right around nightfall.

That’s good news for people with telescopes who don’t want to stay up late. It’s quite convenient to have Jupiter and Saturn highest up for the night as soon as darkness falls. Typically, the view of Jupiter’s four major moons and Saturn’s glorious rings through the telescope is sharper when these worlds are higher up than lower down. The thickness of the Earth’s atmosphere near the horizon tends to blur the view of Jupiter’s moon and Saturn’s rings.

Positions of Jupiter’s moons via Sky & Telescope

Look first for brilliant Jupiter; Saturn is the bright object immediately to Jupiter’s east. Although Saturn is easily as bright as a 1st-magnitude star – as bright as the brightest stars in our sky – the ringed planet can’t compete with the the king planet Jupiter, which outshines Saturn by some 14 times. After all, Jupiter ranks as the fourth brightest celestial object, after the sun, the moon and the planet Venus, respectively.

For the first time since the year 2000, Jupiter and Saturn will showcase a great conjunction in December 2020, for the closest Jupiter-Saturn conjunction since the year 1623. Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe a meeting of the solar system’s two biggest planets, Jupiter and Saturn. The last great Jupiter-Saturn conjunction was May 28, 2000. The next one will be December 21, 2020. But September 2020 presents a fine time to start watching these worlds.

Read more: Before 2020 ends, a great conjunction for Jupiter and Saturn

Watch for the moon in the neighborhood of Jupiter and Saturn for several days, centered on or near September 24.

Venus – the brightest planet – reached its greatest elongation from the sun in the morning sky on August 12 or 13 (depending upon your time zone). But dazzling Venus will remain bright and beautiful as a morning “star” for the rest of this year.

At mid-northern latitudes, Venus rises about 3 1/2 hours before the sun throughout September.

At and near the equator, Venus rises 3 hours before the sun in early September, decreasing to 2 1/2 hours near the month’s end.

At temperate latitudes in the Southern Hemisphere, Venus rises about 2 1/2 hours before the sun in early September, tapering to 1 3/4 hours by the month’s end.

Diagram showing positions of Venus in orbit and its phases at inferior and superior conjunction.

Inferior conjunction – when Venus sweeps between the sun and Earth – happened on June 3, 2020. Some 10 weeks later, Venus reached its greatest elongation in the morning sky on August 13, 2020 (when its disk was about 50% illuminated by sunshine). In September 2020, Venus will start the month about 60% illuminated and then end the month about 71% illuminated. Image via UCLA.

Throughout September, Venus in its faster orbit around the sun will be going farther and farther away from Earth. As viewed through the telescope, Venus’ waxing gibbous phase will widen, yet its overall disk size will shrink. Venus’ disk is 60% illuminated in early September, and 71% illuminated by the month’s end; Venus’ angular diameter, on the other hand, will shrink to 80% of its initial size by late September.

Watch for the waning crescent moon to shine with Venus in the morning sky for several days, centered on or near September 14.

Mercury is an evening planet all month long, though only nominally so at northerly latitudes. September 2020 showcases the best evening apparition of Mercury for the year in the Southern Hemisphere. Mercury will be a whopping 25 degrees east of the sun from September 26 till October 7, 2020, and at its greatest elongation on October 1.

Read more: Mercury in the west after sunset

What do we mean by bright planet? By bright planet, we mean any solar system planet that is easily visible without an optical aid and that has been watched by our ancestors since time immemorial. In their outward order from the sun, the five bright planets are Mercury, Venus, Mars, Jupiter and Saturn. These planets actually do appear bright in our sky. They are typically as bright as – or brighter than – the brightest stars. Plus, these relatively nearby worlds tend to shine with a steadier light than the distant, twinkling stars. You can spot them, and come to know them as faithful friends, if you try.

silhouette of a man against the sunset sky with bright planet and crescent moon.

Skywatcher, by Predrag Agatonovic.

Bottom line: September 2020 presents 4 of the 5 bright solar system planets in the evening sky (Mercury only nominally so at northerly latitudes). Catch Jupiter and Saturn at nightfall, Mars at early evening, and Venus in the predawn/dawn sky.

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Bruce McClure