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.
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.
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.
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.
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.
Bottom line: All you need to know about how to find the bright planets of the solar system during the month of September 2021.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.
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