A diagram of Venus, April 2021. | SkyNews
Venus, April 2021 (Chris Vaughan, Starry Night Education)

Planets at a glance: April 2021

While Jupiter draws farther away from Saturn, the Moon will pass the two April 6-7. Speedy Mercury will also pass by bright Venus on April 25.

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For those of you who like to listen to your stories, click below to hear Susan Trishel Monsøn reading Chris Vaughan’s summary of the planets movements this month.

Mercury

A diagram of Mercury, April 2021. | SkyNews
Mercury, April 2021 (Chris Vaughan, Starry Night Education)

In the opening days of April, magnitude -0.5 Mercury will be visible with difficulty while it sits very low above the eastern horizon before sunrise. The shallow angle of both the ecliptic and Mercury’s orbit will keep the planet in view even as it swings sunward each morning. On April 18, Mercury will pass the Sun in superior conjunction. The steep angle of the western evening ecliptic will rapidly bring Mercury back to visibility above the west-northwestern horizon after sunset for the the final third of April, commencing an excellent apparition in May for Northern Hemisphere observers. On the evenings surrounding April 25, speedy Mercury will climb up the right (northerly) side of much brighter Venus. The best viewing times that evening will be after about 8 p.m. local time. Viewed in a telescope in late April, Mercury will sport a waning, nearly fully-illuminated disk that will be increasing in apparent diameter. It will end the month spanning 5.7 arc-seconds. (Be careful! Ensure that the Sun has completely disappeared below the horizon before using binoculars or telescopes in your search.)

Venus

A diagram of Venus, April 2021. | SkyNews
Venus, April 2021 (Chris Vaughan, Starry Night Education)

Venus will spend all of April in the western sky while it climbs away from the Sun following a late-March conjunction. Venus’ brilliant -3.9 magnitude will allow our sister planet to be seen within the post-sunset twilight after about mid-April. Viewed through a telescope at that time, Venus will show an apparent disk diameter of approximately 9.8 arc-seconds, and a nearly fully-illuminated disk. On the evenings before and after April 25, Mercury will pass much brighter Venus to the right (northerly) side. The best time to see the duo will be at about 8 p.m. local time. (As always, ensure that the Sun has completely disappeared below the horizon before using binoculars or telescopes in your search.)

Mars

A diagram of Mars, April 2021. | SkyNews
Mars, April 2021 (Chris Vaughan, Starry Night Education)

During April, Mars will continue to shine as a bright, reddish dot in the lower half of the western evening sky. Its eastward motion through Taurus, and then through Gemini after April 23, will delay the planet’s descent into the western twilight. Mars will decreases in brightness from 1.3 to 1.56 during April, and its mean apparent disk diameter of 4.9 arc-seconds will also be diminishing. During the middle part of April Mars will form a triangle with the brighter and similarly-coloured stars Betelgeuse and Aldebaran. On April 2, Mars will move closely past the open star cluster NGC 1746, permitting both the planet and the star cluster to be viewed at the same time in a backyard telescope (and in the wider field of binoculars all week long). Mars will have a similar encounter with brighter Messier 35 on April 26. On April 16 and 17, the waxing crescent Moon will hop past Mars. Observers in most of central and eastern Africa, the southern parts of the Middle East, India, Southeast Asia, Indonesia, and most of the Philippines can see the Moon occult Mars at about 12:00 UTC.

Jupiter

A diagram of Jupiter, April 2021. | SkyNews
Jupiter, April 2021 (Chris Vaughan, Starry Night Education)

Jupiter will shine at a bright magnitude -2.1 in the southeastern pre-dawn sky during April. On April 1, the planet will rise among the stars of Capricornus shortly before 5 a.m. local time. After crossing into Aquarius on April 25, Jupiter will begin rising just after 3 a.m. local time. Unfortunately, Jupiter’s eastward motion, and the shallow angle of the morning ecliptic, will prevent the planet from climbing very high before the sky brightens. Telescope views of Jupiter during April will show a large apparent disk diameter that increases from 34.8 to 37.4 arc-seconds. Single transits across Jupiter’s disk by the round, black shadows of its Galilean moons will be commonplace in April. Nearby, fainter Saturn will widen its separation west of Jupiter from 12 to 15 degrees during the month, and the old crescent Moon will pass five degrees south of Jupiter on April 7.

Saturn

A diagram of Saturn, April 2021. | SkyNews
Saturn, April 2021 (Chris Vaughan, Starry Night Education)

During April, Saturn will shine at magnitude 0.7 the east-southeastern sky before dawn, while it travels eastward among the stars of western Capricornus. Saturn will rise at about 4:15 a.m. local time on April 1 and at 2:25 a.m. at month’s end, but the shallow morning ecliptic will keep the ringed planet from climbing high enough to see clearly in a telescope. When viewed through a telescope during April, Saturn will exhibit an apparent disk size that grows from 15.9 to 16.7 arc-seconds. The old crescent Moon will pass less than five degrees south of Saturn on April 6. When combined with bright Jupiter shining about 13 degrees to their lower left, the group will make a nice photo opportunity.

Uranus

A diagram of Uranus, April 2021. | SkyNews
Uranus, April 2021 (Chris Vaughan, Starry Night Education)

After the opening days of April, magnitude 5.9 Uranus will be too faint and too low in the western sky after sunset for viewing, including on April 22-23 when it will be positioned close to Venus and Mercury. Uranus will reach solar conjunction on April 30.

Neptune

A diagram of Neptune, April 2021. | SkyNews
Neptune, April 2021 (Chris Vaughan, Starry Night Education)

Distant blue Neptune, in eastern Aquarius, will be shifting away from the Sun in the eastern pre-dawn sky during April, but the shallow morning ecliptic will keep the magnitude 7.95 planet too low for observing in telescopes until late April, when it will be rising shortly after 4 a.m. local time.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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