These next two weeks bring us the end of Venus’s great show in the evening sky. As Venus sets noticeably sooner after the sun each day, we do get to see the planet pass closely by another planet (the elusive Mercury) — and pair up with a marvelously thin crescent moon. Furthermore, a pair of binoculars or telescope can now show Venus itself as a very slender crescent.
In addition to the planetary events in the evening sky these next two weeks, there is also the Big Dipper and the important stars it points to.
An amazing farewell for Venus: The outstanding sight in the evening sky this spring has been the brightest of all planets, Venus. These recent months have offered not just a display of Venus as the “Evening Star” but the highest display that we get in Venus’s eight-year cycle of recurring appearances. We’ll have to wait another eight years before Venus is once again so high for so long in the evening sky and also for Venus to pass, as it did back on April 3, right through the southern fringes of the beautiful Pleiades star cluster.
Let’s focus, however, on the remaining exciting sights involving Venus that we can look for before the planet passes from the evening sky. First of all, it’s remarkable to see a planet set about 7 minutes sooner after the sun each night. On Tuesday night, Venus sinks below the horizon about two hours after sunset and is easily visible low in the west-northwest in the dusk.
But by Sunday, May 31, the interval between sunset and Venus-set has shrunk to about 25 minutes — probably making even brilliant Venus too low in the sun’s afterglow to glimpse with the naked eye.
An especially amazing sight of Venus to behold now with binoculars or telescope is the crescent phase of Venus. As Venus comes around the curve of the near side of its orbit toward us, its globe appears larger — but the illuminated part appears thinner as Venus gets closer and closer to our line of sight with the sun.
Other wonderful sights involving Venus on certain nights this week include the ”conjunction” (close pairing) of Venus and Mercury and the grouping of the moon with both those planets.
Look low in the west-northwest (where the sun went down) about 45 minutes after sunset this Thursday. That is the night Venus and Mercury are most closely paired, with the much dimmer Mercury shining about 1 degree — less than the width of your little finger at arm’s length — to the lower left of Venus. On Friday after sunset, Venus and Mercury are only a little father apart but with Mercury now to the upper left of Venus.
Then, this Saturday, if you look about 30 to 45 minutes after sunset, and can get an unobstructed view almost right to the west-northwest horizon, you should be able to see a stunningly slender crescent moon glowing not very far to the lower left of Venus. And 24 hours later — on this coming Sunday — the moon forms a line, from upper left to lower right, with Mercury and Venus.
Using the Big Dipper as compass and pointer sign: The Big Dipper is at its highest at nightfall on spring evenings — but appears upside-down. That means that the two “pointer stars” — the ones on the side of the bowl opposite from the Dipper’s handle — now point downward toward Polaris, the star that marks true north. The distance from the pointers to Polaris is about one Big-Dipper-length.
But the Big Dipper is also useful for locating other important stars. If you take the handle of the Big Dipper and extend its curve out about one Big-Dipper-length, it takes you to the brightest star of spring, Arcturus.
A straight line extended onward from Arcturus past the curve from the Big Dipper brings you to Spica, the bright star of the zodiac constellation Virgo. How do you remember all this? With a saying well-known to amateur astronomers: take the arc (from the Big Dipper’s handle) to Arcturus and drive a spike (straight line) to Spica.