When we weren’t looking, Mercury got its foot in the door. Venus has captured our attention these months, shining like a candle in the west at dusk. But if you now look about 15° or a fist-and-a-half below and right of the brilliant planet you’ll see another fainter “star.” That’s Mercury. Although it musters a respectable magnitude 0 (bright as Vega in the Summer Triangle), it’s only about 8° high when the sky gets dark enough to see it.
Find a place with a wide open horizon to the northwest and take along binoculars as insurance. Focus your glass on Venus then sweep the spot where Mercury sits. You should quickly stumble onto a bright, little point of light. Now, lower the binoculars and try to see the planet without optical aid. Mercury will reach its greatest apparent distance from the sun on July 12, so we’ll have it in good view for the next couple weeks as it slowly fades. On July 14 a very thin crescent moon passes directly above the planet in conjunction — a sweet sight in binoculars!
Mercury experiences the most extreme range of temperatures of all the planets in the solar system with a daytime high around 800° F (430° C) and nighttime low of –280° F (–170° C), a range of nearly 1,100° F! Mercury the closest planet to the sun, so it receives more solar energy in the form of heat and light than the Earth. With no atmosphere to retain the heat, the planet radiates the energy back into space at night, and temperatures plummet.
For comparison, the highest temperature ever recorded on Earth was 134.1° F (56.7° C) in Death Valley, California and the lowest was –144° F (–98° C) in East Antarctica in March 2018, a range of 278° F. Earth’s greater distance from the sun combined with the insulating effects of the atmosphere and oceans keep our blue globe from going off the deep end. Let’s hope we can help keep it that way.