Astronomy Essentials

Visible planets – and more – in September 2021

Find these visible planets in September 2021: Venus, Mercury, Jupiter, Saturn, Mars, Neptune

Try Stellarium for a precise view from your location.

For more specific information on planet rise and set times from your location, consult either The Old Farmer’s Almanac (U.S. and Canada) or timeanddate.com (worldwide); or see EarthSky’s list of recommended almanacs.

Visible planets, the moon and more

Venus-Spica conjunction
Look west shortly after sunset in early September 2021 for the Venus-Spica conjunction. Venus is the brightest planet. Spica is the brightest star in the constellation Virgo the Maiden. Around September 4, 5 and 6, 2021, you’ll spot a little star shining near dazzling Venus in the western twilight. That’ll be Spica. Read more.
Person standing watching hazy triangular area of light from horizon to near zenith.
Beginning in early September, start watching for the zodiacal light. From a dark location, look east before dawn begins to lighten the sky. The zodiacal light will appear as a pyramid of light on the eastern horizon, before true dawn. It’s sometimes called the “false dawn.” This eerie light is sunlight reflecting from dust grains that move in the plane of our solar system. You’ll see it in the before dawn from the Northern Hemisphere now, or after all traces of twilight have left the evening sky, in the west, from the Southern Hemisphere, around the equinoxes. Image via our friend Jeff Dai.
Young moon and Venus, and then Antares, in the September 2021 evening sky.
Look west after sunset for the young moon’s return around September 8, 2021. Watch day by day as the waxing crescent sweeps past Venus and moves closer to Antares, Heart of Scorpius the Scorpion. Read more.
Southern Hemisphere view of the young moon and evening planets.
Southern Hemisphere view of the western sky after sunset, beginning with the young moon’s return around September 8, 2021. From the Southern Hemisphere at this time of year, the angle of the ecliptic to the horizon after sunset is very steep. That’s why, for Southern Hemisphere skywatchers, Mercury stays out until after nightfall. Meanwhile, we in the Northern Hemisphere will have a tough time spotting Mercury this month. From northerly latitudes, the shallow tilt of the ecliptic forces Mercury to set below our western horizon shortly after sunset.
A chart showing Venus near the sunset point, and the moon and Antares above it.
The moon pairs up with the star Antares at dusk/nightfall September 12, 2021. The illuminated side of the moon points at Venus near the horizon. Venus is lower in the west than after sunset than Antares now. But Venus will remain in the western twilight for the rest of 2021. Meanwhile, as the September equinox approaches, Antares is now shifting into the sunset glare. Venus will pass Antares, and the two will appear close, in October. Read more.
Neptune, infront of the constellation Aquarius, on line with Saturn and Jupiter.
Here’s one you can’t see without strong binoculars (mounted on a tripod) or a telescope. Neptune comes to opposition in front of the constellation Aquarius the Water Bearer on September 14. A line between Jupiter and Saturn points to Neptune’s location in the sky. The star Fomalhaut – called the Loneliest Star – is also nearby. You’ll need a dark sky for Neptune. Plus, you’ll need a detailed chart; TheSkyLive has one. Read more about Neptune’s opposition.
Star chart with the star Fomalhaut and planets Jupiter and Saturn.
In northern autumn 2021, the bright, solitary star Fomalhaut will appear near the brighter planets Jupiter and Saturn. Look southward from the Northern Hemisphere. Look high in the sky (in southern spring 2021) from the Southern Hemisphere. Read more.
Moon, Saturn, Jupiter September 15 to 18.
The moon passes 4 degrees north of Saturn on September 17, 2021, at about 03:00 UTC. It passes 4 degrees north of Jupiter on September 18 at about 07:00 UTC. On any of these evenings – September 15 to 18, 2021 – the moon can guide you to these 2 outer solar system worlds. By the way, though we show Pluto on our chart, it’s about 1,000 times too faint to be viewed with the eye alone. Read more.
Graphic showing how to use a utility pole, or telephone pole, to see Jupiter's moons.
Utility pole challenge for September 2021, via John Goss/ Astronomical League.
Yellow moon floating in pink sunset sky over blue hills.
View at EarthSky Community Photos. | Here in the Northern Hemisphere, we call the full moon closest to the autumn equinox the Harvest Moon. In 2021, the Northern Hemisphere autumn equinox comes on September 22. The full moon falls less than two days earlier, on September 20. Thus, for the Northern Hemisphere, this upcoming full moon – the full moon closest to our autumn equinox – is our Harvest Moon. Carlos Rios Martinez caught the Harvest Moon from La Bufa Hill in Zacatecas, Mexico, on September 30, 2020. Thank you, Carlos! Read more.
Earth perfectly upright with vertical axis, laft half sunlit, right half in shadow.
The equinox will arrive on September 22, 2021, at 19:21 UTC. That’s when the sun will be exactly above Earth’s equator, moving from north to south. At the equinox, days and nights will be approximately equal in length. For us in the Northern Hemisphere, the sun is rising later now, and nightfall comes sooner. We’re enjoying the cooler days of almost-autumn. Meanwhile, south of the equator, spring is about to begin. Read more.

Planets in September 2021

Also see the indispensable Observer’s Handbook, from the Royal Astronomical Society of Canada

Venus begins September 2021 at a good distance from the sunset (40 degrees from the sun). But for the Northern Hemisphere, the autumn angle of the ecliptic keeps Venus low in the west after sunset. Still, Venus is exceedingly bright. It starts this month at magnitude -3.9. Venus passes 1.7 degrees north of Spica on September 5. The waxing crescent moon passes 4 degrees to its north on the North American evening of September 9. Venus will reach a greatest elongation on October 29 (47 degrees from the sun). But, even then, for us in the north, it’ll hang low in the western twilight. Meanwhile, Venus is glorious this month from Earth’s Southern Hemisphere. Venus will shine brightly in the evening sky for the rest of this year. It’ll reach greatest brilliance as the evening “star” around December 3-4, 2021. Then it’ll shine at magnitude -4.5. Circle early December on your calendar, and see if it’s true that Venus can cast a shadow on a dark night.

Diagram showing positions of Venus in orbit and its phases at different positions.
Venus, and all the planets, travel counterclockwise around the sun. Venus is an inferior planet, or inside Earth’s orbit. So it shows phases like the moon. It swept to the far side of the sun (at superior conjunction) on March 26, 2021, to exit the morning sky and to enter the evening sky. Venus will reach its greatest eastern (evening) elongation from the sun (half Venus) on October 29, 2021. Then on January 9, 2022, Venus will go between the Earth and sun, at inferior conjunction, to exit the evening sky and to enter the morning sky. Image via UCLA.

Mercury continues its best evening apparition of the year for the Southern Hemisphere. For us in the north, the autumn angle of the ecliptic will make Mercury difficult or impossible to see. The Observer’s Handbook describes a “long (relatively) languid loop” for Mercury, east of the sun. The planet comes to its once-in-88-days aphelion (farthest point fron the sun) on September 6. And it reaches greatest elongation east (farthest east of the sun on the sky’s dome, or visible in the west after sunset on September 14 at about 04:00 UTC. At that time, mid-September 2021, Mercury will shine at magnitude 0.1. The thin crescent moon passes 6 degrees to Mercury’s north on September 8. Again, great for the Southern Hemisphere, poor for the Northern Hemisphere.

Jupiter was at opposition on August 19-20. Saturn was at opposition on August 1-2. Opposition marks the middle of the best time of year to see an outer planet. These two are now up in the east at nightfall and are exceedingly prominent nearly all night. Both are bright. Saturn appears as a bright, golden, steady “star,” shining as brightly as the sky’s brightest stars at magnitude 0.3. But Jupiter is much brighter at magnitude -2.6. Jupiter and Saturn are still near each other on the sky’s dome. That’s many months after their great conjunction, when Jupiter passed Saturn in the race of the planets, in December 2020.

A black-and-white diagam showing Jupiter at oppostion (Earth between Jupiter and the sun) and Saturn just past opposition (slightly behind the line between the sun, Earth and Jupiter).
As viewed from earthly north, all the planets orbit the sun in the same direction the sun rotates: counter-clockwise. This illustration of opposition (not to scale) shows the positions of the sun, Earth, Jupiter and Saturn in early August 2021. Saturn reached opposition on August 1-2. Jupiter reached opposition on August 19-20. Image via CyberSky.

Mars is too close to the sun to be seen in September 2021. It will pass behind the sun on October 8. When will you next see Mars? Perhaps in late November 2021, when it begins to emerge in the eastern twilight before sunrise. Afterwards, Mars will have a months-long period of appearing faint and inconspicuous in our eastern predawn sky. But Earth will be steadily plowing ahead of Mars in its smaller, faster orbit around the sun. Inevitably, Earth will catch up to Mars and pass it on the inside track. That will happen next on December 8, 2022. And then Mars will be at its brightest for all of 2022. And it’ll be up all night.

Read more: What to expect from Mars in 2021 and 2022

Neptune reaches its yearly opposition – when Earth flies between this outer planet and the sun – on September 14, 2021. Opposition marks the middle of the best time of year to see an outer planet. And indeed Neptune is generally closest to Earth for the year throughout September. But, because it’s the outermost known major planet in our solar system, it’s still some 4 light-hours away. That’s 28.9 astronomical units (AU), or Earth-sun distances. And thus Neptune is faint. It shines at magnitude +7.8 in September 2021, way beyond the limit for viewing with the unaided eye. You’ll need a dark sky for Neptune. Plus, you’ll need a detailed chart; TheSkyLive has one. And you’ll need strong binoculars securely mounted on a steady tripod, or a telescope. If you can find it, you’ll see that Neptune has a 2.3″ disk this month. That’s minutely larger than usual.

Some resources to enjoy

Try Stellarium for precise views from your location

Try TheSkyLive for precise views from your location

Click here for recommended almanacs to find out precise rise and set times

Translate Universal Time (UTC) to your time

Ecliptic is the sun’s path in our sky

Which ones are the visible planets?

In their outward order from the sun, the five bright planets are Mercury, Venus, Mars, Jupiter and Saturn. These are the planets easily visible without an optical aid. They’re the planets watched by our ancestors since time immemorial. These planets 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 a bright planet and the crescent moon.
Skywatcher. Image via Predrag Agatonovic.

Bottom line: All you need to know about how to find the bright planets of the solar system during the month of September 2021.

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Posted 
September 1, 2021
 in 
Astronomy Essentials

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