Moon, Mercury, stars in Taurus before sunup July 5 to 8
Before sunup July 5 to 8
On the mornings of July 5 to 8, 2021, watch for the waning crescent moon to travel in front of the constellation Taurus the Bull. You’ll see the moon’s position change from day to day as it sweeps closer and closer to the sunrise.
Finally, a very slim crescent will pass the bright yet elusive planet Mercury.
Mercury brighter than Aldebaran
Mercury’s reputation for being elusive doesn’t stem from its being faint. In fact, Mercury is a bright planet, but it’s often seen against a twilight background. The bright twilight makes Mercury appear faint to us. In early July 2021, Mercury is a touch brighter than Aldebaran, one of our sky’s brightest stars. But Aldebaran will probably be the easier of these two starlike objects to spot. That’s because Aldebaran rises before Mercury does. And so you view Aldebaran in a darker sky.
Even when Mercury and Aldebaran are both above the horizon, Mercury will be sitting deeper in the morning twilight glow. So it’ll be harder to see. Remember, though, that the illuminated side of the lunar crescent points to Mercury’s place near the horizon on the mornings of July 5, 6 and 7.
And, remember, binoculars help out immensely in your hunt for Mercury so near the sunrise glare!
Mercury’s rising times
Here are Mercury’s approximate rising time at various latitudes for the next several days, given an absolutely level horizon.
40 degrees north latitude: Mercury rises about 1 1/3 hours (1 hour and 20 minutes) before sunrise
Equator (0 degrees latitude): Mercury rises 1 1/2 hours (1 hour and 30 minutes) before sunrise
35 degrees south latitude: Mercury rises 1 2/3 hours (1 hour and 40 minutes) before sunrise
Aldebaran, Eye of the Bull
The red star Aldebaran – fiery eye of the Bull in the constellation Taurus – is always easy to find in a dark sky. It’s part of a V-shaped star grouping that forms the Bull’s face. This pattern is called the Hyades.
Will you see the Hyades stars around Aldebaran on these July mornings? That’ll depend on your sky. With morning twilight brightening behind Aldebaran, the fainter Hyades stars will be washed out. Still, you might glimpse them if you get up early enough, and if you have a dark sky.
By the way, Aldebaran is an aging star and a huge star! It’s a red giant star. Its computed diameter is between 35 and 40 solar diameters. If Aldebaran were placed where the sun is now, its surface would extend almost to the orbit of Mercury.
The Pleiades star cluster
The glorious Pleiades star cluster – aka the Seven Sisters – is a tiny dipper-shaped cluster. Its stars were born together and still move through space as a family.
The Pleiades is easy to spot in the evening in northern winter. But in July, at mid-northern latitudes, you’ll probably have to be up at least 1 1/2 hours before sunrise, to view the Pleiades in a dark sky. In either the Northern or Southern Hemisphere, dawn’s first light (start of astronomical twilight) precedes sunrise a maximum amount of time at and around the summer solstice.
Beginning of astronomical twilight
Beginning of astronomical twilight at various latitudes (late June/early July):
40 degrees north latitude: Astronomical twilight begins 2 hours (120 minutes) before sunrise
Equator (0 degrees latitude): Astronomical twilight begins 1 1/4 hours (75 minutes) before sunrise
35 degrees south latitude: Astronomical twilight begins 1 1/2 hours (90 minutes) before sunrise
Find out when astronomical twilight begins in your sky via Sunrise Sunset Calendars, remembering to check the astronomical twilight box.
The sky doesn’t need to be quite as dark to see Aldebaran, the constellation Taurus’ brightest star, or Mercury, the innermost planet of the solar system. From most places worldwide, Aldebaran will rise before the start of astronomical twilight. We exclude far-northern temperate latitudes, where the midnight twilight allows for no true nighttime at this time of year.
Southern Hemisphere enjoys the advantage
From temperate latitudes in the Northern Hemisphere, Mercury will rise after the start of astronomical twilight. But, from the Southern Hemisphere, Mercury will come up before the start of astronomical twilight. Moreover, Mercury will rise sooner before the sun at more southerly latitudes, giving the advantage to the Southern Hemisphere for this particular apparition of Mercury in the morning sky.
Far-northern latitudes will have to contend with the early twilight. But mid-northern latitudes – and further south – should enjoy a fine view of the early morning sky in July.
Bottom line: Watch on the mornings of July 5 to 8, 2021, as the waning crescent moon swings by the Pleiades star cluster, the bright star Aldebaran and the bright planet Mercury.