Our sky chart at top is designed for mid-northern N. American latitudes. It shows the western sky at about 75 minutes after sunset on May 30, 2014. The planets Jupiter and Mercury will be similarly positioned in the west at mid-northern latitudes all over the world, although in Europe and Asia, the slender waxing crescent moon will be even closer to the horizon.
We in the N. Hemisphere are enjoying the closing days of a good evening apparition of Mercury, the closest planet to the sun. At mid-northern latitudes, Mercury stays out well over one and one-half hours after sunset. Although, from these latitudes, Mercury might be visible to the unaided eye in a clear sky an hour or so after sunset, it’s a good idea to bring along binoculars, if you have them, in any Mercury search. That’s because Mercury is often obscured in the hazy glow of twilight close to the sunset (or sunrise) horizon.
Mercury is a rocky world with little or no atmosphere, much like our moon. For this reason, both of these worlds get super hot during the day but extremely cold at night.
On the moon, one day lasts for about four weeks, with two weeks of daytime followed by two weeks of night. In the heat of day, the temperate can get as high as 125o Celsius (257o Fahrenheit), and on a cold lunar night, as low as -173oC (-280oF).
But Mercury’s variation in temperature exceeds even that of the moon. On Mercury, the daytime temperature can climb up to 427oC (800oF) and the nighttime temperature as low as -185oC (-300oF). On Mercury, one day (period of time from noon to noon) lasts for 176 Earth-days, or twice as long as one Mercury year of 88 Earth-days.
So as you look for the moon and Mercury in the western sky after sunset, keep in mind that it’s the Earth’s atmosphere that moderates wild swings in temperature on our planet.
Bottom line: On May 30, the waxing crescent moon is near the sun’s innermost planet, Mercury, in the western twilight sky. Mercury is just ending its best evening apparition of this year for N. Hemisphere observers.