This week continues to be an excellent opportunity to observe the elusive planet Mercury. Why is Mercury so hard to observe? After all, it is one of the brightest objects in the sky?
On Wednesday June 12, Mercury will be at its maximum elongation from the Sun. A crescent Moon and brilliant Venus will frame it, making it particularly easy to see. Credit: Starry Night software.
The problem with Mercury is that it never gets very far from the sun. It is the closest planet to the sun, and rarely strays very far away, so most of the time its tiny speck of light is lost in the scattered light surrounding the sun.
Astronomers eagerly await the date when Mercury is at its farthest from the sun, called “greatest elongation.” During the current apparition of Mercury, greatest elongation occurs on Wednesday June 12. On that date, Mercury will be 24 degrees away from the sun.
There is an added difficulty with the current apparition of Mercury for observers in the northern hemisphere. The current angle of the ecliptic, the path of the sun and planets in the sky, puts it very close to the horizon, so that planets like Mercury and Venus, both currently to the east of the sun, never get very high above the horizon.
The good news is that Venus has recently emerged from behind the sun, and now provides a brilliant beacon pointing the way to Mercury.
The trick for finding Mercury is first to find Venus. At a location with a low western horizon, you should be able to spot Venus a few minutes after the sun sets, around 9 p.m. in most locations. Look for Venus slightly north of due west, low on the horizon. Mercury will be just above and to the left of Venus if you’re in the northern hemisphere. I find binoculars very helpful for spotting Mercury, the 7x50 and 10x50 size being especially useful. Focus the binoculars on Venus, then scan above and to its left for Mercury. Don’t wait too late, as Mercury and Venus set quite quickly after the sun.
On Monday June 10, the thin crescent moon will join Mercury and Venus, and the three will form interesting patterns for the next few nights.
In the southern hemisphere, look for Mercury above and to the right of Venus. Because of the angle of the ecliptic, the planets will be much higher in the twilight sky than in the northern hemisphere.
Some of you may be wondering why, if Mercury is the closest planet to the sun, it currently appears farther from the sun in the sky. That’s because of the positions of Mercury and Venus in their respective orbits. On June 12 Mercury is at its maximum elongation from the sun, 24 degrees, while Venus is still far away on the far side of the sun. It won’t reach its maximum elongation from the sun, 47 degrees, until November 1, by which time Mercury will be on the other side of the sun, in the morning sky.