May 17–May 30, 2021
MOUNT WASHINGTON — As twilight deepens tonight, beginning around 9 p.m., locate the crescent moon in the west, mid-way between zenith and the horizon. Below the moon, a juxtaposed pair of bright stars, Pollux and Castor, known as the Gemini twins, stand out. Planet Mars, a rusty-gold point of light, glows rather dimly below Pollux. A ways to the left of Mars, Procyon the Little Dog shines brightly. Balancing Procyon, in the northwest, to the right of Mars, find brighter Capella the Little Goat. Along with Pollux, these two stars shape the last arcing fragment of the Winter Circle, the star pattern composed of winter’s brightest stars. See it poised to disappear from the evening sky as we approach the summer solstice.
When we gaze at Pollux and Castor with the naked eye, we see two single stars. We learn more from the intriguing photograph of glistening streaks at the top of this page. As described by astrophotographer Kent DeGroff, “It is an image of a double star (Castor in Gemini) as it trails for a few seconds through the field of view of a telescope at high magnification… The trailing is caused by the Earth’s rotation with the telescope not tracking. The brighter of the two is referred to as Castor A and the fainter is Castor B, with magnitudes of 1.9 and 3.0 respectively. Actually, there are 6 stars with three being visible (only two in that image) and the others being spectroscopic binaries. The combined brightness of all six stars is +1.6. … If the telescope were moving with the stars, tracking, the double star would be seen as one star, even at high magnification. When the telescope is in a set position, the star is seen moving through the eyepiece.”
Pollux, at magnitude +1.15, is slightly brighter than Castor — the smaller the number, the brighter the celestial object. Recently, a giant planet was discovered orbiting Pollux. Under a dark sky, enjoy Pollux’s golden light in contrast with the blue-white sparkle of Castor.
As the stars of the Winter Circle appear lower to the skyline every night, a drama is unfolding between planets Mercury and Venus. With an unobstructed view to the west-northwest horizon and precise timing, see Venus well before the brilliant planet sets tonight at 9:19 p.m. Mercury (binoculars recommended) follows at 10:07 p.m. On May 28, the two meet in a rather rare planetary conjunction: Mercury sets at 9:40, Venus at 9:41. Look by 9 p.m.
