Sky Talk May 2007: Lunar/Planetary Conjunctions & Earthshine

Conjunctions are close approaches in the sky of the brighter planets, both with one another and also with the Moon. May offers an opportunity to see our lovely satellite successively brush by Mercury, Venus and Saturn in its perpetual orbital motion eastward across the heavens. And as a bonus, in the case of the first two encounters the Moon will also provide an ideal opportunity to view the phenomenon known as “Earthshine.”

At dusk on the evening of May 17th a wafer-thin crescent Moon will pass just 3 degrees north of the planet Mercury (which will be positioned low above the west-northwestern horizon), with closest approach occurring about 8:00 p.m. EDT. Then just two evenings later, on May 19th, the growing crescent Moon passes less than 2 degrees north of the radiant planet Venus, the two worlds being closest around 9:00 p.m.

This will be quite a spectacular pairing since the radiant “Evening Star” will be shining brilliantly at magnitude -4.2, making it the brightest object in the sky after the Sun and the Moon itself! (As will be described in the June installment of Sky Talk, during late May into early July it will actually have become bright enough to cast shadows!) During the above interval, the bright lunar crescent will encompass within it a dimly illuminated disk that’s faintly visible to the unaided eye.

Called “Earthshine,” in the evening sky after sunset we’re seeing sunlight from the Pacific Ocean (where it’s still daylight) being reflected onto the Moon’s surface and then back down to us. In the morning sky before dawn, we’re seeing instead sunlight from the Atlantic Ocean (where it’s already daylight) being reflected onto the Moon and back down to us. The brightness of the Earthshine itself varies with the amount of cloudiness over our oceans at the time.

Both the close conjunctions of the Moon with Mercury and Venus, and the Earthshine itself, are superb sights in binoculars. And while the Earthshine shows up nicely in small telescopes at low magnifications, these glasses typically don’t have a wide enough field of view to encompass both the Moon and planet in the same view. An exception here is Edmund Scientifics marvelous Astroscan wide-field telescope, which provides a whopping 3 degrees of sky coverage (or six full-Moon diameters) at 16x. The spectacle seen through such a telescope of the crescent Moon dimly lit by Earthshine and nearly kissing the dazzling planet Venus is one never to be forgotten!

Just three nights after the Moon-Venus conjunction, on May 22nd, our satellite passes less than a degree north of the beautiful ringed planet Saturn, with closest approach occurring during daylight in the late afternoon. The Moon will be approaching its first-quarter phase and appear half-illuminated. By then, its surface will have become much too brightly lit by direct sunlight to see any sign of the ghostly Earthshine itself. The Astroscan reflector (#30020-01) is capable of showing Saturn’s famed ring system using its 30x eyepiece. And for a really lovely view, observe it with the Edmund Scientifics 60mm refractor telescope (#30823-61) using its 87x eyepiece — which shows Saturn looking like some exquisite piece of cosmic jewelry!

–James Mullaney
Former assistant editor at Sky & Telescope and

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