Tonight – May 28, 2017 – look for the waxing crescent moon to adorn the western evening twilight. Then as dusk gives way to darkness, watch for the bright Gemini stars – whose names are Castor and Pollux – to pop out to the north (or right) of the moon and for the bright star Procyon to pop out to the south (or left) of the moon.
From northerly latitudes, Procyon will set quite early in the evening, so seek for Procyon to the lower left of the moon as soon as darkness falls. From southerly latitudes in the Southern Hemisphere, it’ll be the Gemini stars, Castor and Pollux that set early in the evening, so look for them to the right or lower right of tonight’s moon soon after sunset.
Draw an imaginary line from Procyon and in between the stars Castor and Pollux, as depicted on the sky chart below. At or near the equator, this line (or arc) is close to horizontal, so Castor and Pollux are seen pretty much directly to the right of Procyon.
Yet, at northerly latitudes in the Northern Hemisphere, this line or arc angles upward from left to right, with Castor and Pollux to the upper right of Procyon. The farther north you live, the progressively steeper the angle gets.
At the Earth’s North Pole, this arc of right ascension on the sky’s dome is vertical, or goes straight up-and-down, with Castor and Pollux towering directly above Procyon.
At southerly latitudes in the Southern Hemisphere, this line or arc angles upward from right to left, with Procyon to the upper left of Castor and Pollux. The farther south you live, and the closer you are to the Earth’s South Pole, the closer this line comes to being vertical.
Bottom line: On May 28, 2017, let the waxing crescent moon guide you to the Gemini stars, Castor and Pollux, and to Procyon, the Little Dog star.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.