This is Dudley Observatory’s Skywatch Line for Friday, October 27 through Sunday, October 29, written by Sam Salem.
The evening sky’s sole naked-eye planet is Saturn. However, the 0.5-magnitude planet is losing ground to twilight. As the sky begins to darken, Saturn sits some 15 degrees above the south-southwest horizon where views of its famous ring system will likely be blurred by Earth’s churning atmosphere. Early risers have two planets to enjoy, though neither is much to look at in a scope, at least for now. Mars is first up, rising around 5 a.m., local daylight time. At magnitude 1.8, the red planet is barely brighter than Polaris and has an apparent diameter of just 4 arc seconds—scarcely bigger than distant Uranus! Mars’ brightness and size will greatly increase, but not for many months yet. The second dawn world is Venus. Gleaming at magnitude –3.9, it pops up roughly one hour after Mars. Despite its prominence, Venus is a disappointing telescopic target at the moment, appearing as a small gibbous disc when viewed at medium power. Still, the brilliant planet is a lovely naked-eye sight set against morning twilight.
On Friday, Sun rises at 7:24am and sets at 5:55pm; First Quarter Moon at 6:22pm; Moon sets at 12:00am.
On Friday evening the moon is at or near its first quarter phase, which means the moon’s disk is 50% illuminated by sunshine and 50% engulfed in the moon’s own shadow. First quarter moon arrives on October 27, at 6:22 p.m. EDT
Half the moon is always illuminated in space. In other words, the moon has a day side and a night side, just as Earth does. Due to the angle between the sun, Earth and moon, we’re seeing about equal portions of its day side and night side tonight. Because the moon is now waxing, we’re bound to see more of its day side each evening until the night that the moon turns full on the night of November 3-4.
The part of the moon that isn’t in sunlight is often called the moon’s dark side. Just realize that – because of the moon’s motion around Earth – the portion of the dark side that we see from Earth constantly changes.
There is a permanent far side of the moon. But there is no permanent dark side of the moon, because any given lunar location experiences night for about two weeks, followed by about two weeks of daylight.
The moon does rotate on its axis. But billions of years of Earth’s strong gravitational pull have slowed it down such that today the moon takes as long to rotate as it does to orbit once around Earth. Astronomers would say that the moon is tidally locked with Earth. For that reason, one side of the moon always faces Earth, but it is not always dark – as you can see just by looking at the sky tonight.
Incidentally, the moon’s gravitational effects on Earth are much smaller, but – given billions of years of time – the Earth will slow down and keep one face always toward the moon.
Lunar Straight Wall on Saturday evening. Rupes Recta is a linear fault on the Moon, in the southeastern part of the Mare Nubium at 22.1°S 7.8°W. The name is Latin for straight cliff, although it is more commonly called the Straight Wall. This is the most well-known escarpment on the Moon, and is a popular target for amateur astronomers. 
When the sun illuminates the feature at an oblique angle at about day 8 of the Moon’s orbit, the Rupes Recta casts a wide shadow that gives it the appearance of a steep cliff. The fault has a length of 110 km, a typical width of 2–3 km, and a height of 240–300 m. Thus although it appears to be a vertical cliff in the lunar surface, in actuality the grade of the slope is relatively shallow.
To the west of this escarpment is the crater Birt, which is about 17 km in diameter. Also to the west is the Rima Birt rille. At the southern end is a group of hills often called the “Stag’s-Horn Mountains”, although this name is not officially recognized by the IAU. Northeast of the escarpment is the land cape of Promontorium Taenarium.
To the northeast is the crater Alpetragius, and to the east is Thebit.
From 1969 to 1972, Apollo astronauts had left laser reflectors on the moon’s surface, enabling astronomers to measure the moon’s distance from Earth with great accuracy. Although the moon’s distance from earth varies each month because of its eccentric orbit, the moon’s mean distance from Earth is nonetheless increasing at the rate of about 3.8 centimeters (1.5 inches) per year. That’s about the rate that fingernails grow. Tidal friction with the Earth’s oceans is responsible for this long-term increase of the moon’s distance from Earth. It’s causing the moon to spiral into a more distant orbit. Tidal friction also slows down the Earth’s rotation, lengthening the day by about 1 second every 40,000 years. Hence, the number of days in a year is slowly diminishing over the long course of time. Simulations suggest that at the time of the moon’s formation some 4.5 billion years ago, the moon was only about 20,000 to 30,000 kilometers (12,000 to 18,000 miles) from Earth. Way back then, Earth’s day might have been only 5 or 6 hours long. That would mean over 1,400 days in one year! However, astronomers suspected the moon was receding from Earth before the heyday of the Apollo astronauts. Edmund Halley’s (1656 to 1741) studies of ancient solar and lunar eclipses suggested the possibility, as well. George Howard Darwin (1845 to 1912) is credited for figuring out mathematically how tidal friction affects the moon’s orbit. Studies in fossilized coral indicate that the Earth had spun faster upon its rotational axis when the moon was closer to Earth. Millions of years ago, days on Earth were shorter yet more abundant. For instance, around 900 million years ago, there were about 480 18-hours days in one year. Around 400 million years ago, there were about 400 22-hour days in one year. Looking into the future, astronomers expect longer days but fewer of them in one year.
If the lifetime of the Earth-moon system lasts long enough (which is doubtful), it is projected that after many billions of years, the same sides of the Earth and moon would face one another. In other words, the Earth’s rotational period and the moon’s orbital period would equal one another, representing a period of 47 days. At that time, the Earth/moon distance would expand to some 336,000 miles of 560,000 km, exceeding the present distance of 238,855 miles or 384,400 km by nearly 150%. As you view our companion world tonight, ponder on the rich history and intriguing future of our planet Earth and its wayward moon!
Tonight – just in time for the upcoming season of Halloween and the Day of the Dead – look for the Demon Star in the constellation Perseus the Hero.
That star is Beta Persei, or Algol, pronounced AL-gul. The name Algol comes from an Arabic term for head of the ghoul or head of the demon. This chart showed you how to use the constellation Cassiopeia to locate Perseus in the northeast in the evening sky. The brightest star in Perseus is Alpha Persei, whose proper name is Mirfak.
If you can find Perseus and Mirfak, you can find Algol, too!
Also, as darkness falls this evening, look for the planets Venus and Saturn in your southwest sky.
As darkness falls this evening, look for the planets Venus and Saturn. They’re in the southwest as viewed from Earth’s Northern Hemisphere, more northwest as viewed from the Southern Hemisphere.
Algol is a very interesting star. It’s known to vary in brightness in a way that’s extremely regular. The cycle lasts exactly 2 days, 20 hours and 49 minutes. For a few hours during the cycle, Algol’s brightness falls far below normal, then returns to normal. All the while, the star remains visible to the eye.
Algol’s brightness variation is not due to some special quality of a single star. Instead, this is a multiple star system, where one star regularly passes in front of another as seen from our earthly perspective.
Thus Algol is what’s called an eclipsing variable star.
Thousands of these stars are known, but Algol is perhaps the most famous of this class because its periodic dip in brightness can be seen with the eye alone, and because the cycle is relatively short.
The ancient stargazers had no knowledge of multiple star systems, but possibly they did notice this star’s brightness change. Perhaps the brightness change is why, throughout parts of the ancient world, the star Algol was associated with demons or monsters. The Greeks and Romans identified the star with the Head of Medusa, a fearful monster with snakes in place of hair. The star has also been called the Ghoul Star.
High northern latitudes see Perseus by nightfall or early evening. Observers farther south may have to wait till mid-evening to catch Perseus and the Demon Star, aka Algol, in the northeastern sky.
Bottom line: The best-known star in the constellation Perseus is Algol, its name coming from the Arabic for head of the demon.
The October 27 – 29 weekend features a waxing Moon lighting up the evening sky. So why not spend some time doing a little lunar exploring? There’s lots to see whenever the Moon is up, but on Friday evening the terminator lies near the spectacular crater trio of Ptolemaeus, Alphonsus and Arzachel.
What makes these craters so interesting is that each one has a distinctive appearance. Northernmost is 164-kilometre-wide Ptolemaeus—the biggest of the bunch. Notice its smooth mare-covered floor. Immediately south of Ptoelmaeus is 108-km-diameter Alphonsus, which features a curious north-south ridge. Last is Arzachel, arguably the most conventional of the three. Spanning 96 km rim-to-rim, Arzachel has a classic “complex crater” appearance with its terraced rim and central mountain peak. Also interesting is 40-km-wide Alpetragius, which is nestled between Alphonsus and Arzachel. Alpetragius has an usually large, rounded central mountain. It looks, as lunaphile Charles A. Wood notes, like a nest with a single egg inside. This weekend, take a moment to look in on these craters this weekend and notice how dramatically their appearance changes with increasing illumination.
First U.S. astronomy expedition views eclipse. October 27 1780, the first U.S. astronomical expedition to record an eclipse of the sun observed the event which lasted from 11:11 am to 1:50 pm. The observers left about three weeks earlier, on 9 Oct from Harvard College, Cambridge, Mass., for Penobscot Bay, led by Samuel Williams. A boat was supplied by the Commonwealth of Massachusetts the four professors and six students. Although the U.S. was at war with Britain, the British officer in charge of Penobscot Bay permitted the expedition to land and set up equipment to observe the predicted total eclipse of the sun. The expedition was shocked to find itself outside the path of totality. They saw a thin arc of the sun instead of its complete obscuration by the moon.
October 29 1991, space probe Galileo become the first human object to fly past an asteroid, Gaspra, making its closest approach at a distance of 1,604 km, passing at a speed of 8 km/sec (5 mi/sec). The encounter provided much data, including 150 images, which showed Gaspra has numerous craters indicating it has suffered numerous collisions since its formation. Gaspra is about 20-km long and orbits the Sun in the main asteroid belt between Mars and Jupiter. Gaspra, asteroid 951, was discovered by Ukrainian astronomer Grigoriy N. Neujamin (1916) who named it after a Black Sea retreat. In the photograph (left), subtle color variations have been exaggerated by NASA to highlight changes in reflectivity, surface structure and composition.