During August, as Mercury makes a morning appearance, brilliant Sirius, the sky’s brightest star, makes its first morning (heliacal) rising just before sunrise. For the latitude on the diagram, about 41.7 degrees, this is August 14, 2019. For locations farther south, this occurs days earlier and later for latitudes farther north.
Sirius, the Dog Star, is sometimes called the Nile Star as its heliacal rising historically coincided with the flooding of the Nile River. The Dog Days of Summer (in the northern hemisphere) occur, coincidentally, during August when Sirius and Procyon, the Little Dog Star, appear in the eastern sky before sunrise.
Mercury is at greatest elongation on August 9. Because Mercury is closer to the sun than Earth, we see Mercury appear in either the morning or evening sky around the time the sun rises or sets. It appears in the sky earlier each morning or stays there later each night. It reaches its greatest separation from the sun and then seems to reverse its direction, moving back into sunlight, only to repeat the process a few months later at the other horizon and sky setting.
As Mercury moves back toward the sun in August, it is lower in the sky each morning at about the same time. Sirius appears higher in the sky each morning at the same time. And Mercury gets brighter as it appears nearer to the sun. As Sirius appears higher, it seems to brighten because it gets above the thicker atmosphere that tends to diminish the brightness of celestial objects. They are not quite the same brightness, but appear at the same altitude around August 19.
To locate the planet and the star, find a clear horizon in the east-northeast and east-southeast. Start looking for Mercury and Sirius about 30 minutes before sunrise. A binocular may help in viewing them. Mercury is low in the east-northeast, about 10 degrees up. Sirius is very low, in the east-southeast about 3 degrees up when first visible. Sirius may twinkle wildly this low in the sky. To be sure you have Sirius, don’t confuse it with Procyon in the east and a little higher. Orion is higher in the sky and its three belt stars make an imaginary pointer that take us to the area to find Sirius. Reddish Betelgeuse is higher in the sky, Sirius, Procyon, and Betelgeuse make nearly an equilateral triangle known as the Winter Triangle.
Sirius’ heliacal rising occurs every year about this time. This year the event is a little more interesting because a bright planet is in the sky.