Update: January 4, 2018
Mercury makes 4 appearances in the eastern morning sky during 2018. Two of the more promising apparitions occur in January and December. A third appearance in late August, while not as favorable, presents another view of this elusive planet. During a fourth opposition, in late April, Mercury rises during bright twilight and is very difficult to locate without optical assistance.
This article describes the observing prospects for the apparitions of Mercury.
Mercury in the Morning Sky
The chart above (Figure 2) displays the rising of Mercury and other bright stars and planets in the eastern morning sky compared to sunrise. Moonrise is represented by the circles. The three phases of twilight are graphed as well. The white boxes indicate conjunctions with other stars and planets. The greatest elongations are shown by yellow triangles and the the letters “GE.” The chart is calculated from data computed by the U.S. Naval Observatory. The rising times of Mercury are displayed on the brown curves.
Because Mercury is always near the sun, its appearance starts shortly after inferior conjunction. It rises earlier each day until it reaches its maximum separation from the sun, greatest elongation (shown by the yellow triangles and the letters “GE” on the chart). Then as it moves towards its superior conjunction, its rising time diminishes until it disappears into the sun’s glare.
The best appearances occur around the times of greatest elongation and when the speedy planet arises around the time of astronomical twilight, when the sky is as dark as it gets naturally.
The Mercury Challenge
It is important to note, with these rising time differences shown on the rising chart, that if Mercury rises 1.75 hours before sunrise, it is not visible at that time. This is when Mercury is at the natural horizon. The planet is visible about 30 minutes later; yet, it is low in the sky. The sky is brightening, but still dark enough to see it. Include houses and trees to the scene, you’ll need to find an observing location with a very good horizon.
If the planet rises 45 minutes before sunrise, it is high enough to be seen about 20 minutes before sunrise, after civil twilight. Optical assistance with binoculars or a telescope is necessary to see the planet. As morning twilight brightens, Mercury rises higher in the sky. From Earth, Mercury is always low in the sky shining during twilight. The presence of the Moon, Venus, or Jupiter are helpful to locate it as well.
Viewing Mercury is further complicated by our view of the solar system. As our planet is tilted by 23.5°, the plane of the solar system (called the ecliptic) sometimes makes shallow angles with the horizon. At other seasons, the ecliptic has a higher angle to ease the viewing of the planet. When the ecliptic is highly inclined during autumn mornings, Mercury can appear several degrees higher than at lower inclinations. Usually though, Mercury is about 5° above the eastern horizon before the sky grows too bright. Having a clear horizon is necessary. My viewing position is limited by trees and neighbors’ houses. Sometimes gaps between the trees provide the space necessary to see Mercury when it low in the sky. Many times I walked down the street or view it from nearby park lands.
Mercury is not visible at risings times, as indicated on the chart, but several minutes later. Mercury is almost always visible during some aspect of twilight. In 2018 look for it in January or December. August is fair good, but you’ll need binoculars to a telescope to see it during the April appearance.
Mercury revolves around the sun every 88 days, at the time earth revolves ¼ of its year. Mercury then takes about another earth month to catch up and pass us (116 days total).
Mercury never appears more than 27° from the sun and never appears at midnight from mid-northern or mid-southern latitudes. The chart above (Figure 4) shows Mercury on January 1, 2018, when the planet is 23° west of the sun at its greatest elongation. This chart shows the sun and Mercury at noon, if the sky were dark. The red line represents Mercury’s invisible orbital path.
Hold a ruler (12 inches) at arm’s length. If the sun were at one end, Mercury would appear near the other end at its greatest elongation.
When the rising time of Mercury intersects with other planets or stars on the rising chart, that indicates that the pair rises at the same time, but they are not necessarily closest. For example, the Mercury and Saturn rising lines cross on January 12. The two planets rise at the same time. A conjunction is near, either a few days before or after the intersection date. The conjunction is on January 13 when the two planets are ¾ of a degree apart.
The rising circle for the moon is more predictive than for the planets. For example, on January 15, the moon and Mercury rise about an hour before sunrise. They are 4 degrees apart. Of the moon circle coincides with a rising curve, the pair is close on that morning.
Other conjunctions occur with Mercury’s rising time intersections with Regulus (September 6), and Jupiter and Antares (December 21).
Mercury’s morning appearance begins when this speedy planet passes between the earth and sun. It quickly moves into the morning sky as it moves past and away from us. It rises earlier until it reaches its maximum separation from the sun.
The chart shows Mercury at greatest western elongation when it is at is maximum separation from the sun. This chart shows the sky at noon, if the sky were dark. When a planet is west of the sun, it rises in the east before sunrise.
January 1, 2018 Western Elongation
Update: January 1, 2018, Jupiter, Mercury and Mars
On January 1, Mercury has an excellent apparition with an elongation of 23 degrees. It started with its inferior conjunction on December 12, 2017 (7:48 p.m. CST). While the greatest elongation is only 23 degrees, the planet rises 1.75 hours before sunrise and stands 15 degrees above the eastern horizon at sunrise. Like other elongations, Mercury appears low in the eastern sky as the sky brightens. The ecliptic is inclined with the horizon at about 40 degrees. While the elongation is not at its greatest, the ecliptic’s angle makes a favorable view.
On January 13, Mercury passes less than one degree from Saturn. Jupiter is 44 degrees to the upper right of the conjunction with the moon about midway from Saturn to Jupiter. While dimmer, Mars is about 3 degrees to the lower left of Jupiter and 5 days after its conjunction with the Giant Planet. (See these articles about Mars and Jupiter.)
April 29, 2018 Western Elongation
The April 29, 2017, elongation is the most difficult to see, but it is has the largest elongation of the year, 27 degrees. The apparition begins on April 1 at inferior conjunction (12:53 p.m. CDT). The angle the ecliptic makes is only 25 degrees, and the angle of Mercury’s orbit is less than 20 degrees (See Figure 7). At sunrise, Mercury is only 9 degrees above the horizon. It rises about 50 minutes before sunrise, after nautical twilight, when the horizon is distinguishable from the sky. Look for Mercury with optical assistance, binoculars or a small telescope.
August 26 Western Elongation
This morning appearance begins at inferior conjunction on August 8 (9:06 p.m. CDT). By August 26, the ecliptic is again is inclined about 25 degrees to the horizon as is Mercury’s orbit. Mercury has only an 18 degree elongation, but it rises 90 minutes before sunrise (Figure 8). By nautical twilight the sky is still dark enough to find this planet in the east-northeastern sky.
As Mercury moves towards its superior conjunction it passes about 1 degree from Regulus on the morning of September 6. The diagram above shows the pair about 40 minutes before sunrise when Mercury is only 5 degrees above the horizon (Figure 9). Use binoculars to help with the location of the planets.
December 15 Western Elongation
The final elongation of the year begins with Mercury’s inferior conjunction on November 27 (4:15 a.m. CST). Mercury reaches its greatest angular separation on December 15; this elongation is 21 degrees. It stands nearly 20 degrees above the eastern horizon at sunrise. While the elongation is small, Mercury’s orbit is inclined over 50 degrees with the horizon (Figure 10). Mercury rises just before the beginning of twilight. When the planet reaches 5 degrees in altitude, the sky is still fairly dark, yet beginning to brighten.
Nearly a week after its greatest elongation, Mercury passes less than 1 degree from Jupiter on the morning of December 21 (Figure 11). Jupiter passes its solar conjunction on November 26 and slowly emerges into morning twilight on its apparition that takes over a year. In the chart above, Mercury is about 4 degrees above the horizon.
The rising chart above shows Mercury and Antares rising at the same time on December 28, but Mercury is nearest Antares on the same morning as it in conjunction with Jupiter.
By the time Antares rises high enough (5 degrees) the sky is brightening and the diagram shows the triad about 35 minutes before sunrise (Figure 12).
Mercury is the most difficult planet to see among the 5 naked eye planets. Because it is so close to the sun, it appears low in the sky during twilight. During 2018, Mercury makes four morning appearances. The apparitions in January and December are easily observed. The August apparition is fairly easy to see, but the April appearance in the morning sky needs optical help.