Click through this short slide show to see Venus, Mercury and the Moon on March 21, 2018.
Mercury makes 3 evening appearances in the western sky during 2018. Because the planet is always near the sun, it rarely appears in a dark sky. In some instances it sets minutes after the end of twilight. Most of the appearances occur in bright twilight (as seen in Figure 2 above) when the planet sets during twilight.
The setting chart above (Figure 2) shows the setting times of planets, stars and the moon (circles) compared to sunset. The three phases of twilight are displayed as well.
Mercury zips around the sun every 88 days. If Mercury starts at inferior conjunction, it rapidly moves around the sun, reaching that starting point in less than three months. Because of its shorter path and higher speed, Mercury overtakes us in an additional 28 days. It’s fast! It seems to bounce from morning sky to evening sky rapidly. Some apparitions of Mercury are easier to seen than others.
Mercury’s evening apparitions begin when it passes behind the sun (superior conjunction) (Figure 3). Near conjunction, Mercury is invisible to us under typical daylight conditions because it is near the sun and in our sky during the daytime. After conjunction it sets later each day and gradually brightens. Beginning about two weeks after conjunction it starts appearing low in the western sky during evening twilight. (Figure 2) Binoculars may be needed. As it heads towards its greatest elongation it brightens and sets later. As the planet gets closer to us, it brightens. Unlike Venus, it is impossible to predict its greatest brightness interval because of shadows on the surface from mercurian features and other factors. For Venus, its cloud tops present a consistent featureless face — allowing greatest brightness predictions.
After greatest elongation, Mercury sets earlier during brighter twilight as it moves between our planet and the sun.
It is important to note the settings on the setting chart on Figure 2, Mercury is visible before these times. On March 15 (Figure 4), the planet sets nearly 100 minutes after sunset. Look for the planet 30 minutes before the setting times. Mercury will be low in the sky. The monthly passing of the moon or periodic appearance of other planets aid in its location. Binoculars help with the initial observation. Then try to locate it without their assistance.
The best appearances occur Mercury sets near the end of (astronomical) twilight when the ecliptic has a high angle compared to the horizon. On other apparitions, the angle of the plane of the solar system (ecliptic) makes with the horizon is shallow. One those occasions Mercury is very low in the sky, setting around the time of nautical twilight. Even when Mercury has a wide elongation (27 degrees) a shallow inclination of the ecliptic with the horizon makes Mercury set sooner. During summer twilight lasts about 30 minutes longer than during spring. A shallow ecliptic inclination and a long twilight makes Mercury spotting challenging during summer.
Autumn apparitions are limited by the extremely shallow ecliptic inclinations.
Spring evenings are the best times for spotting Mercury, if it above the horizon during the evenings at that season. This year we have such an appearance.
March 15 Elongation
The first and best elongation for seeing Mercury begins with superior conjunction on February 17 (6:27 a.m. CST). Mercury quickly moves into the evening sky. Brilliant Venus is in the vicinity and helps finding speedy Mercury. At its greatest elongation, Mercury is only 18 degrees from the sun. At sunset (Figure 5), it stands about 16 degrees above the horizon and sets nearly 100 minutes after sunset. At this time Venus is 4 degrees from Mercury and both fit into a binocular field. Then once the sky is dark enough, look for Mercury without optical assistance.
Update: Venus and Mercury, March 3, 2018
Update: This conjunction has been added to the original article — As Venus and Mercury emerge from superior conjunction Mercury passes about one degree from Venus on March 3. The planets are low in the western sky, about 5 degrees above the horizon at 30 minutes after sunset. Venus is 5 times brighter than Mercury. It may be necessary to first locate Mercury with binoculars.
Three days after greatest elongation Venus and the crescent moon help in locating Mercury. Brilliant Venus is 3.75 degrees to the lower left of Mercury and the moon is nearly 4 degrees from Venus (Figure 6). All should fit into the same binocular field.
Update: Venus, Mercury and the crescent moon, March 18, 2018.
Mercury rapidly moves back into twilight and between Earth and the sun. Inferior conjunction occurs April 1 (12:53 p.m. CDT).
July 12 Elongation
By summer Mercury reappears in the evening sky, after its superior conjunction on June 5 (9:02 p.m. CDT). The angle of the ecliptic is shallow and Mercury is only 13 degrees above the horizon at sunset when the planet reaches greatest elongation on July 12.
Brilliant Venus is 16 degrees to the upper left of Mercury at this time. Mercury sets about 90 minutes after sunset. Twilight ends 130 minutes after sunset in mid-July. Because of the lengthy twilight, look for Mercury with binoculars.
As Mercury moves towards its greatest elongation, it passes 4.75 degrees Pollux on June 24, though Mercury is only 4 degrees above the horizon when the sky is dark enough to see it. Venus is 19.5 degrees to the upper left of Mercury.
After greatest elongation, Mercury moves between the earth and sun on August 8 (9:06 p.m. CDT) and into the morning sky.
November 6 Elongation
The final 2018 evening appearance of Mercury begins with superior conjunction on September 21 (8:52 p.m. CDT) It slowly appears in the west-southwestern sky as it heads toward its greatest elongation (23 degrees).
While at a favorable angular separation from the sun, Mercury stands only 8 degrees above the west-southwest horizon at sunset, setting less than one hour after the sun. Clearly this appearance is difficult to see. The ecliptic has a shallow inclination and Mercury appears below it. Seeing Mercury requires binoculars and a very clear horizon free from clouds and terrestrial features. While a challenge, look for these events with binoculars:
Mercury passes 3 degrees from bright Jupiter on October 28. Locate Jupiter with binoculars, Mercury is in the same field to the lower left of the bright planet.
While low in the sky, Mercury passes 2 degrees from Antares on November 9, while a thin crescent moon shines 6 degrees above Mercury. View the moon through binoculars. Position the field of view so that the moon appears at the top of the field. Mercury appears at the bottom of the field. Jupiter is about 9 degrees to the right of Mercury A clear view of the horizon is necessary to see this collection of celestial gems.
Mercury’s best evening appearance occurs during the spring. Brilliant Venus helps locating the speedy inner planet. The other two appearances require good horizons and some optical aid with binoculars. Happy observing!