Lunar X on Thursday Evening
A few times a year, for a few hours around the First Quarter Moon, a feature called the Lunar X becomes visible in strong binoculars and small telescopes. When the rims of the craters Parbach, la Caille, and Blanchinus are illuminated from a particular angle of sunlight, they form a small, but very clear and bright X shape. It’s located on the terminator about one third of the way up from the southern pole (bottom) of the Moon (at 2° East, 24° South). The prominent round crater Werner sits to the lower right.
The next Lunar X will form in daylight after 5:40 pm Eastern Time on Thursday, September 8th, peak around 7:40 pm, and last until at least 8:40 pm. The Moon will be nicely positioned in the sky, but due to twilight, the X won’t be strongly visible until after the peak.(Above: The Lunar X sits a third of the way from the left, along the terminator. Smartphone image through a small telescope by Tanya Oleksuik)
The Moon and Planets
The Moon begins this week as a slim young crescent visible above the western evening horizon. As the evenings pass, the Moon will climb away from the sunset, until it reaches First Quarter. The four main phases of the moon, New, First Quarter, Full, and Last Quarter, all occur when the Moon sits in its orbit at specific angles from the Sun: directly towards the Sun for New, opposite the Sun for Full, and at right angles to the Sun for the two quarters. The moments when it reaches those angles can occur any time of the day or night – not just when the Moon is up.
First Quarter moons are half lit, and always appear in the evening sky. This month’s First Quarter Moon occurs at 7:49 am Eastern Daylight Time on Thursday morning. So the Wednesday evening Moon will be almost 50% illuminated, and Thursday evening’s Moon will be a bit more than 50 % illuminated. Remember, the evenings surrounding the First Quarter are the best times to view the Moon with binoculars or a telescope because the sunlight is striking the terminator (the pole-to-pole line separating the light and dark sides) nearly horizontally. The brightly lit mountain peaks and crater rims cast deep black shadows, and new zones are highlighted every evening as the Moon waxes fuller and the terminator travels across its face.
Thursday night also sees the Moon situated only a few degrees (three finger widths) above yellowish Saturn, while brighter, red Mars sits well off to the east (left). The twinkling star Antares, the brightest star in Scorpius the Scorpion, is about a palm’s width below Saturn. I’ll post a diagram here. While the Moon’s visit is temporary, the two planets are above the southwestern horizon every evening, setting about 11 pm local time. Keep your eye on Mars. Every night it shifts a little to the left, towards the Milky Way and the Teapot of Sagittarius the Archer.Above: The First Quarter Moon visits Saturn and Mars on Thursday, September 8, shown here at 9 pm. Each evening, Mars is sliding eastward (left) towards the Milky Way and the teapot in Sagittarius. Star Walk 2 app simulation.
Venus is the very bright object low in the western sky after sunset. It sets about 8:30 pm this week. Uranus and Neptune are in the eastern evening sky. Uranus, between the fishes of Pisces, rises about 9 pm local time, while Neptune, in Aquarius the Water-Bearer is already up at dusk. Due to their extreme distance, they change location only a little from week to week.Above: Venus, shown at 8 pm, will be sitting above the western horizon for many weeks as it slowly climbs away from the Sun. Star Walk 2 app simulation