This is Dudley Observatory’s Skywatch Line for Friday, October 23, through Sunday, October 25, written by Sam Salem.
On Friday, Sun rises at 7:19am and sets at 6:00pm; Moon rises at 2:43pm. Moon completes the first quarter of its orbit around Earth at 9:23am on Friday. The evenings surrounding first quarter are the best ones to see the lunar terrain when it is dramatically lit by low-angled sunlight. For a few hours near its first quarter phase, a feature on the Moon called the Lunar X becomes visible in strong binoculars and backyard 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 obvious X-shape. At approximately 10pm on Friday, the Lunar X is predicted to peak in intensity. The phenomenon will be visible for approximately two hours on either side of that time. On Saturday evening, the pole-to-pole terminator that divides the lit and dark hemispheres of the waxing gibbous Moon will fall just to the left of Rupes Recta, also known as the Straight Wall. The rupes, Latin for “cliff”, is a north-south aligned fault scarp that extends for 65 miles across the southeastern part of Mare Nubium, which sits in the lower third of the Moon’s Earth-facing hemisphere. The feature is always prominent a day after first quarter and day before last quarter. For reference, the very bright crater Tycho sits due south of the Straight Wall.
Venus, at magnitude -4, crosses from Leo into Virgo on Friday morning. The planet is visible in the three hours before sunrise, slowly rising higher in the sky as daylight breaks. Look to Venus’ north and southeast to find Nu (ν) and Beta (β) Virginis, respectively. Together, the three objects form a triangle with the stars creating its wide base.
On Friday, the dwarf planet Ceres will complete a retrograde loop that began in July, causing it to temporarily cease its motion through the background stars. On this night, the magnitude 8.6 object will be located in the lower part of the southern evening sky, about a fist’s diameter to the upper right of the very bright star Fomalhaut, in constellation Piscis Austrinus, the southern Fish.
On Friday night, turn your attention across the sky to the west, where you’ll find asteroid 11 Parthenope right at the corner of constellation Pisces, just 1.5 degrees north-northeast of star Alrescha. The magnitude 9.5 asteroid was, as its number suggests, the 11th body discovered in the asteroid belt. Annibale de Gasparis is credited with that discovery in 1850. Parthenope reaches opposition on Friday morning at 10am.
The Moon may have kept our planet’s atmosphere safe from a more active Sun, 4 billion years ago, with a magnetic field that has long since disappeared. While the Moon has no magnetic field of note today, recent evidence from rock samples brought back by the Apollo missions show that between 4.2 and 3.4 billion years ago, when the Moon was more than twice as close to Earth as it is now, it did have a magnetic field that was at least as strong as Earth’s present magnetic field. A team in NASA used this information to model the interaction of the early Moon’s magnetic field with Earth. They found that the magnetic fields of the Moon and Earth should have combined to create a protective magnetosphere. Several million years later, as the Moon drifted away from Earth and its core cooled, its field died. The combined field would solve a key problem with the young Earth. Scientists believe the Sun was more active in the early life, ejecting up to 100 times more solar particles than now, making prospects for life on Earth bleak. But instead life flourished. According to NASA scientists, confirming their model could have implications in the hunt for life beyond our solar system. If terrestrial exoplanets have large moons, they may have produced the same kind of protective effect.