Catch a Glimpse: Mercury, the Moon, and the Gemini Twins in the Night Sky

As the nights continue to lengthen and the stars begin to twinkle, there's a celestial event that you won't want to miss. On July 10, observers in the Eastern Time zone will have the opportunity to see Mercury and the moon near Pollux and Castor, the bright "twin" stars of Gemini. This is a rare chance to catch a glimpse of this fast-moving and smallest major planet, which orbits the sun 4.15 times a year but appears to go around only 3.15 times from our vantage point. During the first week of July, we'll have a good chance at sighting Mercury, which swings rapidly back and forth relative to the sun in our sky while moving along in an orbit scarcely more than half as far from the sun as Venus. This year's second evening apparition, which runs from July 10 through July 24, is an excellent time to view Mercury against a dark sky as it sets just after the end of evening twilight. This time around, it's early summer rather than late winter, so the duration of evening twilight is longer by about 30 minutes from mid-northern latitudes. Unlike in early March, we can now hope to glimpse Mercury against a dark sky. However, during its evening appearances, Mercury always appears brightest in the days leading up to its greatest elongation. On July 10, it shone at magnitude -0.2, brighter than the similarly-hued star Arcturus, the second brightest behind Sirius as seen from northern latitudes. By July 24, it will have faded to a still respectably bright magnitude of +0.5, which would rank it among the top ten brightest stars in the sky.

On Thursday evening, July 11, Mercury will form a nearly straight line with two bright stars; the Gemini Twins, Pollux and Castor. Going from right to left, the distance between Castor and Pollux measures 4.5 degrees. If you were to draw an imaginary line between these two stars and extend it another 6.5 degrees to the left, you will come to Mercury. In addition, keep in mind that Mercury will appear much brighter than either Pollux and Castor. On this night, Mercury will shine at zero magnitude, while Pollux will appear only about a third as bright at magnitude +1.1 and Castor only about one quarter as bright at magnitude +1.6. So while you'll likely be able to see Mercury with your naked eye, Pollux and Castor will likely be more difficult to see amidst the twilight glow. Be sure to have a pair of binoculars on hand to help you see these Twin Stars teamed up with Mercury.