Suddenly, we’ve gone from one planet to three at dusk. Used to be only Mars. Then Jupiter joined the party (rising in the southeast in late twilight) and now Mercury makes it a trio. The closest planet to the sun is only about half again as big as our moon and like it, peppered with craters and airless. Mercury sometimes looks pinkish to the naked eye because it assumes the rosy color of dusk, but it’s a grayish place and hotter than blazes with an average daytime temperature of 800° F (427° C).
Look for the planet low in the northwestern sky for the next three weeks. Tonight’s appearance starts with a bang thanks to a stunning crescent moon only 6° (three fingers held together at arm’s length) to the left of the planet. Depending on your location, the moon’s age ranges from 39 hours for observers on the East Coast to 42 hours for those on the West. Either way, it’s a sublime sliver.
To see the pair, find a place with a good view of the horizon (or as near as you can get) to the west-northwest. Then, get your local time of sunset here. Add a half hour to that, and that should be the time to start looking for the pair. I usually can spot Mercury and a low moon about 40-45 minutes after sunset in a relatively haze-free sky. Smoke from Canadian forest fires will pose a problem for some places, so watch out for that. You’ll know smoke’s an issue if the sun is dim and blood-red around setting time, in which case seeing either object could be a challenge.
Both Mercury and the crescent will stand about 5° high when they first emerge in twilight. The bright moon will be easy to spot and make an excellent pointer to Mercury, a short distance to its right. If you have any difficulty use binoculars.
In the coming nights, the moon quickly vaults upward from the horizon (to the east), fills out and becomes much easier to see. Mercury does the same only more slowly. It inches upward in the next two weeks fading some while doubling its altitude to 10°. This will be the last good evening apparition of the planet this year for northern hemisphere skywatchers. While Mercury will return to the evening sky this fall it will be too low to see well, so now’s the best time to make its acquaintance.
Apparitions occur about every 2 months and alternate between dusk and dawn. The cycle is a reflection of Mercury’s orbit around the sun and how we see it from our perspective on Earth. Because it’s close to the sun the planet never strays far from it, the reason it only appear during twilight closely following or preceding the sun. When its orbit takes it to one side of the sun we see it in the morning sky as a “morning star.” When it swings around to the opposite side, it appears at dusk as the “evening star.”
During this apparition Mercury will pass very close to fainter Mars on the nights of June 17 and 18 with closest approach of just 17 arcminutes (a little more than half a full-moon-diameter) on the 18th. I hope it’s clear on that future date because my forecast for tonight is rain — grrrrr. I hope you’ll fair better. Clear skies!