Moon and Taurus before dawn August 23-25

Unless you’re a night owl, you probably won’t see the moon and the constellation Taurus the Bull climbing up into your sky before your bedtime. Your best view will be before dawn, or before the beginning of astronomical twilight, on August 23, 24 and 25. That’s because the moon and Taurus climb highest up just before daybreak.

Visit Sunrise Sunset Calendars to find out when dawn’s first light (astronomical twilight) begins in your sky, remembering to check the astronomical twilight box.

The chart at top is designed for mid-northern North American latitudes. Even so, the moon will still be passing in front of this constellation as seen from around the world. Before dawn on these dates in the world’s Eastern Hemisphere – Europe, Africa, Asia, Australia and New Zealand – look for the moon to be offset toward the previous date. The farther east you live, the greater the offset.

On the morning of August 23, the moon is at or near its half-illuminated last quarter phase, as it passes to the south of the Pleiades star cluster. The following day, August 24, finds the moon sweeping to the north of Aldebaran, the brightest star in the constellation Taurus.

The lit side of a waning moon always points eastward, or in the moon’s direction of travel through the constellations of the zodiac. Relative to the backdrop stars, the moon travels eastward at the rate of about 1/2 degree (the moon’s own angular diameter) per hour. Hence, from around the world, the moon will appear closer to the star Aldebaran on August 24 than on August 23.

Star chart of constellation Taurus with ecliptic, Elnath, and Aldebaran marked.

When the moon is far enough north of the ecliptic, it can occult the Pleiades star cluster – or even the star Elnath. On the other hand, when the moon resides south of the ecliptic, it can occult the star Aldebaran. When the moon passes through Taurus, it can swing anywhere from 5 degrees north to 5 degrees south of the ecliptic, depending on where the moon is positioned in its 18.6-year cycle.

Aldebaran, the star depicting the eye of the Bull, crowns a V-shaped assemblage of stars that outlines the Bull’s face. Extend this starlit V upwards to Elnath, Taurus’ second-brightest star. Elnath marks the tip of the Bull’s northern horn. The fainter star at the tip of the Bull’s southern horn is Zeta Tauri. This 3rd-magnitude star, though rather faint, is easy to see on a dark, moonless night. We don’t see the this star, Zeta Tauri, on the feature sky chart at top because it’s covered over by the August 25th moon, but you can see it on the IAU chart of Taurus immediately above – or, below, where we remove the August 25th moon.

Star chart with moon on 23rd and 24th, Taurus, ecliptic, Elnath, and Aldebaran.

The star below Elnath, the northern horn star, is Zeta Tauri, the southern horn star. If you live along the West Coast of the U.S. and Mexico, you might be able to see an occultation of this star before dawn on August 25, 2019.

World map with ovals in Pacific Ocean and West Africa.

Every place between the solid white lines can see the lunar occultation of the star Zeta in a nighttime sky. The short blue lines show where the occultation happens at dawn, and the dotted red lines depict where the occultation takes place in a daytime sky. Worldwide map via IOTA (International Occultation Timing Association).

By the way, if you live along or near the Pacific Coast of the United States and Mexico, you can watch the moon occult (cover over) the star Zeta Tauri before dawn on August 25. Zeta Tauri will disappear behind the moon’s illuminated side and then reappear on the moon’s dark side.

We give the occultation times for two localities in local time:

San Diego, California
Occultation begins (Zeta Tauri disappears): 3:43:34 a.m. PDT
Occultation ends (Zeta Tauri reappears): 4:39:49 a.m. PDT

Guadalajara, Mexico
Occultation begins (Zeta Tauri disappears): 5:34:20 a.m. CDT
Occultation ends (Zeta Tauri reapppears): 6:42:07 a.m. CDT

Visit lunar-occultations.com to find out the occultation times for nearly 700 localities, but remember to convert from Universal Time to your local time. Here’s how.

Bottom line: Before dawn these next several mornings – August 23, 24 and 25, 2019 – watch for the waning moon to cross the constellation Taurus the Bull.

Bruce McClure