2020, August 15-16: Heliacal Rising of Sirius

2020, August 15-16: The heliacal rising of Sirius is the star’s first appearance in the morning sky before sunrise.

The helical rising (annual first appearance) of Sirius in the morning sky is a spectacular sight.  During 2020, this occurs in mid-August.

(Reported sighting of Sirius from 33.8° north latitude with a binocular, August 6, 2020.)

(Author saw Sirius, with a binocular through a broken  cloud deck, 29 minutes before sunrise on August 12, 2020. To see it without optical assistance in a few mornings, see the description below.) 

(August 16:  Sirius easy to see 48 minutes before sunrise without a binocular.  Sky was cloudy before  sunrise on August 15.)

2020, August 14: Sirius clears the horizon 38 minutes before sunrise. The star is visible through binoculars and a short time exposure, but not with the unaided eye.2020, August: Sirius is visible, without a binocular, in the east-southeast, 46 minutes before sunrise.

by Jeffrey L. Hunt

The spectacular appearance of a bright star in the morning before sunrise is an impressive sight. While very low in the sky, the star twinkles against the brightening hues of morning twilight.

The first morning appearance of a star in the eastern sky before sunrise is known as the “heliacal rising” of the star.

The first morning appearance of Sirius attracts attention. The brightest star in the sky, it can be found near the horizon before we see other bright stars.

In the lore of earlier generations, the heliacal rising of Sirius was thought to cause “dog days.” It’s coincidental that the “Dog Star” first appears in the morning sky during the hot days of the year.

Observing the first morning appearance of a bright star is a challenging feat. This requires a perfectly clear sky to the horizon and a vantage point to see the natural horizon, free from trees, buildings, houses, and other obstructions.

A Sky and Telescope magazine article described the circumstances of the date of the heliacal rising of Sirius. The author described that when the sun is about 8° below the horizon and Sirius is 3° in altitude in the east-southeast sky, the star should be first visible. For this writer’s latitude (41.7° North), no single date meets the criteria. The best pair of days is August 15, 2020, and the following morning. On the former morning, Sirius is slightly lower than 3° and on the morning of the latter it is slightly higher than the visible limit.

The chart above shows the sky 42 minutes before sunrise on August 16, 2020. Bright Venus and the crescent moon are high in the east.

Betelgeuse, Procyon, and Sirius – while they are part of their own constellations – make a large equilateral triangle, known as the Winter Triangle. The trio is prominent in the evening sky during the colder months in the Northern Hemisphere.

Procyon is sometimes translated as “before the dog.” It rises about 25 minutes before Sirius, so it rises before the Dog Star.

For beginners, start looking in the morning sky about August 12. Locate Betelgeuse and Procyon. A binocular may help you initially find the stars. Venus is nearly above Procyon, although the planet is much higher in the sky. On the diagram, Procyon is only 8° in altitude; that’s about one-tenth of the way up in the sky from the horizon to overhead (zenith). Betelgeuse is higher, about one-third of the way up in the sky, at about the same altitude as brilliant Venus. Once you see the two stars, you can visualize the scale of this large celestial triangle.

After you recognize Procyon and Betelgeuse, look each clear morning to continue to find the visible pair. Then scale the other two sides of the Winter Triangle, Betelgeuse – Sirius and Procyon – Sirius, and attempt to look for the nighttime’s brightest star very low in the east-southeast sky.

For observers north or south of this writer’s location, shift the heliacal rising date one or two days earlier for the southern United States and similar latitudes. Add one to two days for locations farther north.

When do you first see Sirius? Respond in the comments section of this article.

Read the Venus as a Morning Star, 2020-2021 article.