To find your way around the sky, it often helps to have a guide, whether it be a book, a star-finder, or an easily recognisable constellation. But there’s no better guide than Earth’s natural satellite, which rounds the celestial sphere once a month. This October, make the trusty old Moon your astronomical tour guide.
Lively evenings as the month begins
We start the month with a new moon, “visibly” absent from the night sky. It makes a grand entrance during the hour following sunset on the evening of October 3: Scan the southwest horizon for a thin lunar crescent hovering above the dazzling planet, Venus.
Did you know you can see the unlit part of the Moon’s disk when it’s a thin crescent? This otherwise dark region, which faces away from the Sun, appears brighter than the background sky because the Earth reflects sunlight onto it. Known as lunar “ashen glow”, the best time to observe this striking phenomenon is from three to five days before, or after, the new moon.
The Moon’s course carries it toward the planet Saturn, which it encounters in the constellation of Ophiuchus on October 5 & 6. Though less radiant than it was this summer, the beautiful ringed planet is still the brightest object in this region of sky, outshining Antares, the red star at the heart of the Scorpion.
If you’re searching for the other red “star” that accompanied Saturn, in Scorpius, over the past few months, you’ll need to look at Sagittarius instead. This fall, Mars moves “full-throttle” through the constellations above the southern horizon. If you want to find the tiny Red Planet easily, the Moon will pass above it on October 7 & 8.
Bright nights during mid-month
As the nights go by, the Moon moves along its orbit toward the east; our natural satellite reaches first quarter on October 9, and as it grows brighter, darkness gives way to moonlit nights.
Before October 16, when the Moon is opposite the Sun and its face is fully illuminated, it will pass near two challenging planets (actually, impossible to see with the naked eye) making them easier to spot in binoculars or a telescope.
Neptune is first to receive a lunar visit during the night of October 12 to 13. Around 2:00 a.m., before both objects set, the Moon and the 8th planet will be separated by just one lunar diameter; close enough to be visible in the same telescopic field of view. Though the Moon will help locate Neptune, its brilliance makes the observation more difficult. Center the planet, and you’ll see a faint blue dot shining in the diffuse glow of moonlight off to the right. That tiny dot is actually the giant gaseous planet, which measures 16 times the Moon’s diameter, but is located over 10,000 times (4.3 billion kilometers) farther away.
On October 15, Uranus is next to encounter the Moon, in the constellation of Pisces. On that date, Earth and the turquoise giant planet will be at their closest approach of the year. Though Uranus is right at the limit of naked-eye visibility, optical instruments will be needed to observe the 7th planet, due to the presence of a nearly-full moon shining just a few degrees beneath it.
While the Moon passes close to several planets this month, the bright star, Aldebaran, will be completely eclipsed by the lunar disk. The waning lunar crescent will move through the constellation of Taurus from October 17 to 19, occulting its brightest star from 1:56 to 2:46, on the morning of October 19.
Shining in twilight at month’s end
As the Moon pursues its course, it rises progressively later, becoming a dawn object toward the end of the month; this provides another opportunity to observe the lunar ashen glow.
On October 28, an hour before sunrise, a very thin lunar crescent will appear above the eastern horizon with brilliant Jupiter, visible at dawn until the end of the year, shining just to its right. A twilight conjunction of these two objects is always spectacular, but it will be absolutely magnificent to see using binoculars or a telescope. In a one-degree field of view, even at low magnification, 5 moons will be visible: ours, and the four Galilean satellites of Jupiter.
For those who are not early risers, the evening twilight will offer beautiful sunsets and, from October 27 to November 3, a series of conjunctions of Venus and Saturn, shining in Ophiuchus near the red star, Antares.
The Moon will accompany the planetary duo in November, but not before completing its monthly cycle and disappearing for a few days surrounding the new moon, which occurs on October 30. So rest assured: any werewolves that you meet at month’s end will just be “halloweeners” dressed in their customary attire.