Mercury’s best evening appearance of 2021 occurs during May. The planet is visible in the west-northwest above Venus after sundown. The appearance begins during late April. On May 28, the apparition ends with a close Venus – Mercury conjunction, the closest easily visible until 2033. About 30 Venus – Mercury conjunctions occur during the interval, but some close groupings are lost in the sun’s glare.
by Jeffrey L. Hunt
Mercury is the planet closest to the sun. It moves very rapidly along its orbital path. It is less than half the earth’s distance from the central star. It whips around the sun every 88 days, shuttling from one side of the sun to the other, appearing in the western sky after sunset or in the eastern sky before sunrise.
The speedy planet catches Earth and passes it every 116 days.
During spring evenings and autumn mornings, the plane of the solar system makes a high angle with the horizon at mid-northern latitudes. If Mercury is placed appropriately in its orbit, the planet races into the sky and is visible in the glow of twilight. The planet is rarely seen in a dark sky with no twilight.
At most Mercury can be 27° from the sun. Even with its maximum separation, known as the greatest elongation, the planet’s appearance can be difficult to view, if not placed appropriately for a spring evening view or autumn morning appearance.
During 2021, brilliant Venus assists in locating the elusive planet.
As Mercury sprints into the evening sky, it is brighter than the brightest star Sirius, although this occurs when the planet is in bright twilight.
Here’s what to look for:
April 25: Twenty minutes after sunset use a binocular to find Venus about 3° up in the west-northwest. Mercury is 1.2° to the upper right of the planet.
Each evening Mercury appears higher above Venus, but a binocular is needed to see it.
April 28: Mercury is 3.0° above Venus at 20 minutes after sunset.
May 1: Thirty minutes after sunset, Mercury is 5.2° above Venus.
Each evening, the planet is easier to see with the unaided eye in a darker sky, and each evening it is dimmer.
May 8: Thirty minutes after sunset, Mercury is 8.5° to the upper left of Venus.
The moon moves through the region on May 12 and May 13.
May 12: The moon and Venus make their closest pairing of this evening appearance of Venus. The gap between them is 1.2°.
May 13: Forty-five minutes after sunset, the razor-thin crescent moon is 3.3° to the upper left of Mercury. The planet is bright enough to be seen without a binocular.
The planet’s best evening is May 16, shortly before it reaches its greatest elongation.
May 16: Mercury is 8.7° to the upper left of brilliant Venus and 5.4° below Elnath, the Northern Horn of Taurus. Mercury’s brightness is fading each evening. It is bright enough to locate without a binocular.
Each evening Mercury is to the upper left of Venus and near Elnath.
May 20: Forty-five minutes after sunset, Mercury is 7.0° to the upper left of brilliant Venus and 3.6° to the lower left of Elnath.
Each evening the two planets are low in the west-northwest, the gap between them decreases each evening. Mercury’s brightness continues to dim as well. Use a binocular to view the planets and the starry background.
May 25: Mercury is 3.3° to the upper left of Venus and 4.4° to the lower left of Elnath.
May 27: Mercury is 1.2° to the upper left of Venus.
May 28: Mercury is 0.4° to the lower left of Venus. Use a binocular to find the dimming planet with brilliant Venus. This is the closest easily observed Venus – Mercury conjunction until November 5, 2033, although about 30 groupings of the two planets occur during the interval.
Mercury retreats into the sun’s brightness, reaching its inferior conjunction on June 10. The planet quickly moves into the morning sky, although the summer appearance occurs during latter evening twilight.
The next evening apparition is a dismal prospect at its best. It sets less than an hour after sunset, before the sun is 12° below the horizon.
The year’s best morning appearance occurs on September 13 when Mercury is nearly 27° from the sun, rising at the beginning of morning twilight.
Read more about Venus in our summary document.
Read more about the planets during May 2021.
October 9, 2021: Look for brilliant Venus, crescent moon, and the head of Scorpius in the southwest after sunset. About every eight years, Venus and the moon appear near the head of the Scorpion after sunset. Look for them about 45 minutes after sunset.
October 1, 2021: Before sunrise, the lunar crescent is near the Beehive star cluster.
Newly released analysis from the Mars Reconnaissance Orbiter shows that the Arabia Terra region on Mars experienced powerful volcanic eruptions.
September 30, 2021: An hour before sunrise, the crescent moon is near the Gemini Twins.
September 29, 2021: The thick crescent moon is in the southeast before sunrise, approaching the middle of Gemini. The evening planet pack is visible after sunset.