Special to CosmicTribune.com, August 19, 2024

Excerpts from weekly Sky&Telescope report.

SUNDAY, AUGUST 18

■ Whenever bright Vega shines nearest your zenith, as it does right after dark now. Two hours later, when Deneb passes the zenith, it’s the turn of little Delphinus and boat-shaped Capricornus down below it to stand at their highest due south.

After those two, the third star of the Summer Triangle is Altair. It’s the one three or four fists above the Moon early this evening.

MONDAY, AUGUST 19

■ Full Moon. The Moon is exactly full at 2:26 p.m. EDT, so for the Americas it rises during bright twilight after the Sun has set. As night comes on, watch for Saturn to become visible to its left or lower left, by about a fist and a half at arm’s length.

This is a blue moon by the original definition: the third full Moon in a season that contains an unusual four (Maine Farmer’s Almanac definition, 1937). It’s not a blue moon by the simpler and more common definition nowadays: the second full Moon in a calendar month.

This is also a supermoon: a full Moon that happens when the Moon is near perigee, making it appear about 7% or 8% larger than average — barely enough to be discernible if you’re a practiced Moon watcher. (The opposite, when the full Moon is near apogee, has been called a minimoon.)

TUESDAY, AUGUST 20

■ Saturn shines very close to the just-past-full Moon this evening. They’re only about 1° apart or less as seen from North America. Their background is dim Aquarius.

WEDNESDAY, AUGUST 21

■ Arcturus is the brightest star high in the west on late-summer evenings. In astronomy lore nowadays, Arcturus may be best known for its cosmic history: It’s an orange giant some 7 billion years old, older than the solar system, racing by our part of space on a trajectory that indicates it was born in another galaxy: a dwarf galaxy that fell into the Milky Way and merged with it.

But in the astronomy books of our grandparents, Arcturus had a different claim to fame: It turned on the lights of the 1933 World’s Fair in Chicago, celebrating “a century of progress.” Astronomers rigged the newly invented photocell to the eye end of big telescopes around the US and aimed them where Arcturus would pass at the correct moment on opening night. When the star’s light crept onto the photocells, the weak signals were amplified and sent over telegraph wires to Chicago, and on blazed the massive lights to the cheers of tens of thousands.

Why Arcturus? Astronomers of the time thought it was 40 light-years away (modern value: 36.7 ±0.2 light-years). So the light would have been in flight since the previous such great event in Chicago, the World’s Columbian Exhibition in 1893.

And earlier? Arcturus was famous as one of the first stars discovered to show proper motion, its own independent motion on the celestial sphere. In 1718 Edmond Halley realized that Arcturus, Sirius, and Aldebaran had moved more than half a degree from where the Greek astronomer Hipparchus had carefully measured them to be some 1,850 years earlier.

And before that? Arcturus was the first nighttime star to be seen in the daytime with a telescope: by Jean-Baptiste Morin in 1635.

THURSDAY, AUGUST 22

■ As summer progresses and Arcturus moves down the western sky, the kite figure of Boötes that sprouts up from it tilts to the right. The kite is narrow, slightly bent, and 23° long: about two fists at arm’s length. Arcturus is its bottom point where the stubby tail is tied on.

The Big Dipper now slants at about the same height in the northwest, to the Kite’s right.

FRIDAY, AUGUST 23

■ The brightest star high in the southeast these moonless evenings is Altair, with little orange Tarazed above it by a finger-width at arm’s length.

A little more than a fist-width to Altair’s left is delicate Delphinus, the Dolphin, leaping left.

Above Altair, slightly less far, is smaller, fainter Sagitta, the Arrow. It too is pointing leftward. You’ll need a nice dark sky.

SATURDAY, AUGUST 24

■ August is prime Milky Way time when the Moon is out of the evening sky. After dark now the Milky Way runs from Sagittarius in the south, up and left across Aquila and through the big Summer Triangle very high, and on down through Cassiopeia to Perseus low in the north-northeast.

■ The Moon, nearly last quarter, rises around 10 or 11 p.m. Once it’s well up an hour or two later, look for the Pleiades about a fist at arm’s length to its lower left.

SUNDAY, AUGUST 25

■ Last-quarter Moon tonight; exactly so at 5:26 a.m. Monday morning. The Moon is now a few degrees lower left of the Pleiades, and in the more distant vicinity of much else, as shown above.

■ And, Algol in Perseus shines two fists upper left of the Moon (off the chart above). Here you can catch Algol doing its famous act: It should be in one of its self-eclipses, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 3:13 a.m. EDT Monday morning August 26th; 12:13 a.m. PDT. Algol takes several additional hours to fade and to rebrighten.