Let the Moon guide you through the May sky!
Early May will be a great time to behold our companion in all its splendour: Indeed, the Moon will be full on May 5 at 1:34 p.m. However, you need to take some precautions when observing it with an optical instrument, to avoid being blinded by its intense brightness. After all, with a diameter of 3,475 km and an average distance from Earth of 380,000 km, the Moon is like a big mirror that reflects sunlight back to us. And even if this mirror is grey and dull, devoid of bright clouds or vast bodies of water, the moonlight—concentrated through a telescope, spotting scope or even regular binoculars—can hinder night vision. There are special eyepiece filters that reduce the Moon’s brightness to a more comfortable level, allowing you to observe its entire disc. You’ll get to see Selene in all its glory and explore the multiple greys of its seas and oceans, in search of the deepest craters ringed with white.
When the Moon is full this month it will lie in Libra, the constellation that precedes Scorpius along the ecliptic (the apparent path of the Sun and planets on the celestial sphere). Scorpius is easier to recognize than inconspicuous Libra thanks to its characteristic shape and magnificent alpha star Antares, whose orange hue and brightness resemble those of Mars. In fact, its name derives from the Ancient Greek “anti-Ares,” meaning “rival of Mars.” The Moon will sit mere degrees from Antares on the night of May 6-7; the pair can be found above the southeastern horizon at about 11 p.m.
On the lookout for earthshine
Let’s continue exploring the sky with the Moon as our guide. It reaches its last quarter phase on May 12. As of the 16th, early birds will have the chance to admire a progressively thinner lunar crescent. It will also be an opportunity to observe the Moon’s ashen glow, also known as “earthshine”: The part of the Moon not directly illuminated by the Sun but rather by sunlight reflected from Earth’s clouds and oceans. This earthshine is much fainter than direct sunlight, however, so when the sunlit portion of the lunar disc is too large (first quarter, last quarter, gibbous or full Moon), earthshine simply gets swallowed up in the Sun’s glare. Therefore, you can only catch earthshine when the Moon is a thin crescent. On May 17 at dawn, about 30 minutes before sunrise, the crescent Moon will hang just 1½ degrees to the right of Jupiter, very low on the eastern horizon.
After the New Moon of May 19, the lunar crescent reappears in the evening sky as of the 20th, after sunset and at nightfall. You’ll have several evenings to admire its accompanying earthshine. On May 22 and 23, the crescent Moon will be very close to the brilliantly glowing Venus; look for the duo in the west as soon as twilight sets in and until about 11 p.m.
The month is coming to an end, but the Moon continues to be our guide among the planets and constellations. It can be found a few degrees above Mars on the evening of May 24, due west in the first half of the night. On May 27, the first quarter Moon shines below Leo’s belly.