Monday, May 3, 2021

The brightest planets in May's night sky: How and When to see them

4:31 PM |

Three planets adorn the evening sky and two in the predawn morning sky. In the evening, Mars continues to draw away from Earth and grows progressively dimmer. It now ranks with stars of the second magnitude and spends much of this month in Gemini in the general vicinity of the stars Pollux and Castor.

Meanwhile much brighter and much lower in the west-northwest sky is Venus, struggling to work its way up out of the sunset fires of evening twilight. There will be a rather close conjunction of Venus with a hairline crescent moon on May 12, a lovely sight if you can catch it. Mercury is out there too, a bit higher than Venus, though much dimmer. Yet, this will be its best evening apparition of 2021.

In the morning sky, for the third consecutive spring, we see Jupiter and Saturn together again, although unlike the previous two years, they are now slowly drawing apart. The moon will pay them a visit on the morning of May 4 and again with Saturn on the morning of the Memorial Day holiday.

In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them.


Mercury — the smallest planet, has its most favorable apparition of this year for mid-northern observers this month. Except for the final week of May, this reddish planet is an easy naked-eye object at dusk; an extraordinarily good opportunity. Find a spot with a clear view to the west-northwest and start looking a half hour after sunset. The planet will get easier to see as twilight deepens. Mercury begins this month quite brilliant, shining at magnitude -1.1. But as the illuminated portion of its disk decreases, it fades by about 0.1 magnitude per day to +1.5 on May 24. If you're at latitude 40 degrees north you'll find Mercury at least 10 degrees high in the west-northwest sky 45 minutes after sunset from May 10 through May 22. Against the background stars, Mercury will be 16 degrees to the lower right of Aldebaran on May 1; 2 degrees south of the Pleiades (visible in binoculars) on May 3; and 8 degrees north (upper right) of Aldebaran on May 9 and May 10. On May 13, an extremely slender (4% illuminated) crescent moon appears about 3 degrees to the left of Mercury. They're low in the west-northwest at dusk, setting before the sky is fully dark. Mercury reaches greatest elongation, 22 degrees east of the sun on May 17, and is visible in a completely dark sky. This rocky little world has faded to magnitude +0.5, and its tiny disk is only 35% lit. The planet fades rapidly in the following days, becoming too faint to observe by May 25. On May 28, dropping rapidly back toward the sun, Mercury will pass very close — just 0.4 degrees to the left — of Venus. This would have been a striking conjunction if it had happened two or three weeks earlier, when Mercury was much brighter. But it now shines at magnitude +2.2, more than 250 times dimmer than Venus and is all but impossible to see against the backdrop of the bright twilight sky even in binoculars. It will arrive at inferior conjunction with the sun on June 10th.


Venus — is a rather difficult sight as May begins, setting only 45 minutes after the sun for viewers at mid-northern latitudes. The planet's visibility improves considerably during the month, however. By May 31, Venus's altitude at sunset has increased from 8 degrees to 14 degrees and the bright world doesn't set until about 1 hour 25 minutes after sundown. Soon after sunset on May 12, an exceptionally thin (1% illuminated) lunar crescent might be glimpsed about 1 degree to the lower left of Venus. Use binoculars to accentuate your chances of getting a view of this rather striking tableau.


Mars — is in Gemini, fairly high in the west at dusk, 11 degrees below Pollux and Castor. It passes 5 degrees south (lower left) of Pollux on May 31. In a telescope the Red Planet is a featureless little dot. At magnitude +1.7, it now glows only 1/53 as bright as it did last October. On May 15, Mars can be found at nightfall, sitting about 1.75 degrees to the upper left of the waxing crescent moon.

Jupiter And Saturn

Jupiter and Saturn are early morning objects. Check out the southeast sky on May 4 at around 4 a.m. local daylight time and you'll see a broad isosceles triangle formed by the moon, Jupiter and Saturn. The moon-Jupiter/moon-Saturn sides measure 9 degrees, while Jupiter and Saturn are separated by 15 degrees. Jupiter is in western Aquarius, rising around 3 a.m. daylight saving time on May 1 and shortly after 1 a.m. on May 31. During the month Jupiter brightens from magnitude -2.2 to -2.4, and its disk grows 7% in apparent width at the equator.

The big planet comes to western quadrature (90 degrees west of the sun) on May 21. Can you see that the planet's western limb is slightly less illuminated than its eastern limb this month? Saturn in Capricornus rises by 2:30 a.m. local daylight time on May 1 a.m. and 12:30 a.m. on May 31. But it's not well placed for observing until shortly before the first light of dawn; a splendid ball and rings tilted 16.8 degrees to our line of sight. On Memorial Day morning (May 31) the waning gibbous moon sits 5.5 degrees to the lower left of Saturn during the predawn hours.

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