The young moon swung close to the planet Mercury on Saturday evening, July 14, 2018. And it was spectacularly near the much-brighter planet Venus on Sunday evening, July 15. By Monday – July 16 – the moon has moved above Venus in the western twilight sky. They’re not as spectacular as on Sunday, but, still, very beautiful.
The pair are fairly high up in the west after sunset, easiely viewed by all who have a clear sky. When you see them, be sure to notice the soft glow of earthshine – twice-reflected sunlight – on the nighttime side of the moon.
If you could see Venus through a telescope now, you’d find the planet in a waning gibbous phase. But, unlike a waning moon, Venus is actually getting brighter to the eye, night by night, not fainter. Venus’ greatest brightness doesn’t occur at full phase but when its disk exhibits a crescent phase (about 25 percent illuminated by sunshine). That happens because, in order to appear to us as a crescent, Earth and Venus have to be on the same side of the sun in the solar system. Venus has to be nearly on a line between us and the sun, in order to appear in a crescent phase. And that means Venus is relatively close to us when it’s a crescent. Venus will reach its greatest brilliancy as the evening “star” in September 2018. Thus, although Venus is now waning in phase, the sky’s brightest planet is also brightening day by day.
Mercury is a different story. It’s fainter than Venus to start out with and fainter now than it was some days ago.
Like Venus, the phases of Mercury are only visible through the telescope. But, like Venus, Mercury’s phase impacts this planet’s overall magnitude (brightness). Mercury’s maximum brightness happens near full phase and minimum brightness occurs near new phase. But Mercury’s visibility also depends on this world’s angular separation from the sun. Mercury is presently 26 degrees east of the setting sun.
No matter where you were on Earth on Saturday evening, you would have needed clear skies and an unobstructed horizon in the direction of sunset to see Mercury. That’ll be true Sunday and Monday evenings, too … Mercury will be there, below the moon and Venus. But it’ll be much harder to see than either the moon or Venus, especially from the Northern Hemisphere.
The Southern Hemisphere has a much better view of Mercury now. Mercury stays out later after dark at southerly latitudes. For instance, at mid-northern latitudes, Mercury sets about one and one-quarter hours after sundown; at the Earth’s equator (0 degrees latitude), Mercury sets about one and three-quarter hours after the sun; and at temperate latitudes in the Southern Hemisphere, Mercury sets more than two hours after sunset.
Want to know Mercury’s present elongation from the sun? Click here and look under the last column on the right, entitled solar elongation. You can also seek out Mercury’s apparent magnitude in the middle column.
Day by day, Mercury is dimming in the evening sky because this world is waning toward new phase. Mercury will reach new phase on August 9, 2018, to transition out of the evening sky and into the morning sky.
Click here for a sky almanac giving you Mercury’s setting time in your sky.
By the way, both Venus and Mercury are inferior planets; they orbit closer to the sun than Earth does. And it might seem odd to you that one of these planets, Mercury, is brightest at full phase (100 percent illuminated) while the other, Venus, appears most brilliant in our sky at about 25 percent illumination. Apparently, this difference is due to the fact that Venus is covered over by clouds whereas Mercury is a rocky world much like our moon.
What will you see? The only way to know is to look!
The two tweets below are both from Friday night, via Ian Griffin (@IanGriffin on Twitter) – director of the Otago Museum in New Zealand. He caught the extremely young moon on Friday:
Tonight the moon was 27 hours hold when I spotted it setting across the harbour. Here’s a picture. At the time just 1.8% of its surface was illuminated by the sun. This is a real tribute to the transparency of the Otago Sky! #moon #beauty pic.twitter.com/S2oOaYFvTR
— Ian Griffin (@iangriffin) July 14, 2018
Ian also caught this great sun pillar Friday, followed by the planet Venus:
— Ian Griffin (@iangriffin) July 14, 2018
It’s possible to see many wonderful things, if you look!
Bottom line: Monday evening – July 16, 2018 – is your last chance this month to see the moon and planet Venus relatively close. After Monday evening, the moon – pursuing its easterly orbit around Earth – will be waxing larger and shifting away from Venus.