2021, May 15: Planets on Review

May 15, 2021: Five planets and the crescent moon are visible today.  Morning Planets Jupiter and Saturn are in the southeast before sunrise.  Brilliant Evening Star Venus, Mercury – on its best evening appearance of the year, Mars, and the crescent moon are in the western sky after sunset.

by Jeffrey L. Hunt

Chicago, Illinois:  Sunrise, 5:30 a.m. CDT; Sunset, 8:04 p.m. CDT.  Check local sources for sunrise and sunset times for your location.

Morning Sky

The planet parade continues.  The five brightest planets are visible today.  Jupiter and Saturn are in the morning sky, while Venus, Mercury, and Mars are in the western sky after sunset.

The sun is between the two groups. Its brilliant glare interrupts seeing all five in the sky at the same time.

This morning, Saturn rises in the east-southeast over four hours before sunrise.  Bright Jupiter follows less than an hour later.

By an hour before sunup, both are low in the southeastern sky. Saturn, brighter than most of the stars in the sky, is over 25° above the south-southeast horizon.  In the starfield, the planet is 0.6° to the right of Theta Capricorni (θ Cap).

Saturn is a week away from the beginning of its retrograde motion, when the planet appears to move backwards (west) compared to the background stars.

Each day the sun, moon, and planets appear to rise in the east and set in the west.  This is from our planet’s rotation.

While moving westward, the sun, moon, and planets appear to move eastward compared to the starry background.

The moon’s eastward motion is evident from night to night.  If you have observed the moon this week, you have seen it higher in the western sky as it moves toward the east compared to the planets in the western sky.

On occasions, the planets seem to stop their eastward motion against the stars and appear to move backwards.

All the planets retrograde, but the outer planets are most evident and easily observable.

As Earth moves faster around the sun, it catches up to and passes between the sun and the outer worlds, the distant planets seem to stop moving eastward and start moving westward.  Then as Earth moves away from the worlds, they stop moving westward and begin to move eastward again compared to the starry backdrop.

Saturn’s eastward motion compared to the star Theta Capricorni is slowing and stops in a week.  Then the planet slowly beings its westward pattern.

Jupiter, brighter than all the stars in the morning sky, begins to retrograde in over a month. The Jovian Giant is 16.8° to the lower left of Saturn.  As Saturn slows and begins its retrograde path, the gap between the two planets continues to widen.

About five months ago, the two planets were at their great conjunction in the southwestern sky after sunset.

Jupiter is moving eastward in Aquarius.  The stars in the constellation are faint, famous constellation, dim stars.

Use a binocular to observe the Jovian Giant 1.7° to the upper left of Iota Aquarii (ι Aqr) and 4.8° to the lower right of Theta Aquarii (θ Aqr).

Saturn and Jupiter set during daylight hours.

Evening Sky

After the sun leaves the sky, three planets and the moon are visible in the western sky.  Thirty minutes after sundown, brilliant Venus is nearly 6° up in the west-northwest. 

Find an observing location with a clear horizon.  A hillside facing the west-northwest or an observing spot from an elevated structure will assist in seeing a clear horizon.

You may initially need to use a binocular to see Venus.

Mercury, nearing its greatest separation from the sun, is nearly 9° to the upper left of brilliant Venus.  Mercury is dimming each evening as it is easier to see.  Take the optical assist here with he binocular to pick out the planet from bright twilight at this hour.

The crescent moon, 15% illuminated, is over one-third of the way up in the west. Dim Mars is not labeled on the chart above.  It is 2.3° to the upper left of the lunar slice.  Both fit into the same binocular field, but at this sky brightness, the planet is a challenge to see.  Attempt to see it with the binocular.

Venus as an evening star article.

Read more about Venus in our summary document.

By 45 minutes after sunset, Mercury is easier to spot in the sky. At this hour, the speedy planet is over 12° above the horizon.  The brighter stars appear at this twilight level.  The crescent moon and Mars are visible, but Venus is quite low in the sky.

Here’s more about Mercury during May 2021.

An hour after sunset, Venus is near the horizon, Mercury is to its upper left, while the moon and Mars are in Gemini.  Castor and Pollux are above Mars and the moon.

Notice the night portion of the moon is gently illuminated by earthshine.  Sunlight reflects from Earth’s oceans, land, and clouds to brighten the moon’s nighttime.

This is similar to the moon illuminating the terrestrial landscape.  From the moon this evening, Earth is a bright gibbous phase in the sky.

Here’s more about Mars during 2021.

Find a detailed chart of the motion of Mars during the month here.

This evening, Venus sets 66 minutes after sunset.  Mercury is at its latest setting interval from sunset.  It sets 118 minutes after sundown.  Mars follows the two inner planets to the horizon less than fours after sunset.  The parade begins tomorrow morning, about 90 minutes after Mars sets.  Saturn appears above the east-southeast horizon.

Detailed Note: Five planets are visible during a 24-hour interval. Two are visible before sunrise and the remaining three after sunset.  One hour before sunrise, Saturn is over 25° above the south-southeast horizon.  It is 0.6° to the right of θ Cap.  In a week the Ringed Wonder stops moving eastward and begins to retrograde.  Since May 1^st, Saturn has moved only 0.3° eastward compared to the sidereal background.  As the planet slows to begin its apparent westward motion, Jupiter continues to open the gap to Saturn.  This morning Jupiter – over 22° up in the southeast – is 16.8° of ecliptic longitude east to Saturn and to its lower left. In the starfield, Jupiter is slightly to the upper right of an imaginary line that runs from ι Aqr to θ Aqr.  The Jovian Giant is 4.8° to the lower right of θ Aqr and 1.7° to the upper left of ι Aqr. The Pleiades star cluster rises at sunrise. Rigel, “Orion’s left knee,” sets at sunset.  This is known as the star’s Cosmic Setting. The display of bright planets and the crescent moon continue in the western sky after sunset.  During its best evening appearance this year, Mercury (m = 0.2) nears its greatest elongation.  Thirty minutes after sunset, brilliant Venus is nearly 6° up in the west-northwest.  Use a binocular to locate Mercury – over 14° above the horizon – 8.9° to the upper left of Venus.  The moon (4.3d, 15%) is over 33° in altitude above the west horizon.  Fifteen minutes later, Venus is over 3° in altitude and Mercury is nearly 12° up in the west-northwest.  Mars (m = 1.7) is 2.3° to the upper left of the lunar crescent. Venus is closing the gap to Mars approaching their summer conjunction.  This evening, Mars is 35.2° of ecliptic longitude east of Venus. The gap has closed over 8° since May Day.  One hour after sunset, Mercury is 9.0° above the horizon and 6.3° to the lower right of Elnath. (Venus sets 66 minutes after sunset.) Mars is 3.5° to the upper left of ε Gem, while the moon is 1.4° to the upper left of the same star.  The Red Planet is 3.7° to the upper right of ζ Gem.

Read more about the planets during May 2021.