Sky at a Glance 2021 August 14 – 21

Photo depicting the constellation Cepheus the King (house shaped--laying on its side) in the northern sky above the W-shaped Cassiopeia.

This Week’s Sky at a Glance, 2021 August 14 – 21 ~by Curt Nason

The constellation Cepheus the King is quite large but it can be difficult to pick out. Around 9:30 pm, look northward for a group of five moderately bright stars in the shape of a house on its side and situated above the W-shape of Cassiopeia the Queen. The peak of the house is only about a fist-width to the right of Polaris, the North Star, and the constellation lies just below a line from Polaris to Deneb at the tail of Cygnus the Swan. A colourful star can be seen in binoculars or a scope just below the base of the house. Herschel’s Garnet Star, a red supergiant, is one of the most luminous stars known and is a thousand times wider than the Sun. If placed in the middle of our solar system it would stretch beyond the orbit of Jupiter.

Another famous star in Cepheus is Delta Cephei, which is situated near the bottom left of the house; it being the namesake of the Cepheid variable stars. Such giant stars pulsate with a regular period and subsequently dim and brighten consistently over that time. For example, Delta Cephei dims and brightens by a factor of two over five days. Early in the 20th century, Harvard astronomer Henrietta Swan Leavitt discovered that the intrinsic brightness of a Cepheid variable was proportional to its period and worked out a formula for this relationship. Using the 100-inch telescope on Mount Wilson in the 1920s, Edwin Hubble detected Cepheid variables in what was then called the Andromeda Nebula. Knowing the intrinsic brightness of these stars based on their periods, and on how stars dim with distance, he determined the distance to these stars and proved that the nebula was actually a galaxy outside of the Milky Way.

In mythology, Cepheus and Cassiopeia were the rulers of Ethiopia. Poseidon had made a ferocious sea monster to ravage the land as punishment for Cassiopeia’s boasts of their daughter Andromeda’s beauty. To get rid of the monster, they chained Andromeda to the rocks at the seashore as a sacrifice to the monster. She was rescued by Perseus, whose namesake constellation is seen below Cassiopeia.

This Week in the Solar System

Saturday’s sunrise in Moncton is at 6:17 am and sunset will occur at 8:29 pm, giving 14 hours, 12 minutes of daylight (6:24 am and 8:32 pm in Saint John). Next Saturday the Sun will rise at 6:26 am and set at 8:17 pm, giving 13 hours, 51 minutes of daylight (6:32 am and 8:20 pm in Saint John).

The Moon is at first quarter this Sunday and it will be below Saturn and Jupiter next Friday and Saturday, respectively. You will need binoculars and a clear sky to see Mercury and Mars a third of a Moon-width apart on Wednesday, setting just 40 minutes after sunset. They will be about a hand span to the lower right of brilliant Venus. Jupiter is at opposition on Thursday, proudly revealing its stormy Red Spot to telescope users around 10:30 pm. Saturn is 15 degrees west of Jupiter, within the chevron-shape of Carpricornus.

Questions? Contact Curt Nason.

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