The return of Sirius signals seasonal change

Orion and Sirius: Star chart: Morning sky in August with Sirius and constellation Orion above it and stars labeled. — Here’s an early morning sight you won’t want to miss. The return of Sirius and the winter constellations to the morning sky heralds a change of season. Chart via John Jardine Goss/ EarthSky.

In late August and early September, look for two hints of the changing season in the predawn sky: Orion the Hunter and Sirius the Dog Star. Recognizable for the short straight line of three stars that make up his Belt, Orion rises before dawn at this time of year. And the sky’s brightest star, Sirius – also known as the Dog Star, as it’s part of the constellation Canis Major the Greater Dog – follows Orion into the sky as the predawn darkness gives way to dawn.

Have you noticed a very bright, madly twinkling star in this part of the early morning sky? Many do – around the world – at this time of year. That star is Sirius. It’s so bright that, when it’s low in the sky, it shines with glints of red and flashes of blue. That’s the effect of our thick atmosphere causing its light to waver, or twinkle.

You won’t see Orion and Sirius in the evening sky until northern winter (or southern summer). But presently, the Hunter and the Dog Star lord over the sky at dawn.

Stars and constellations pass behind the sun

Orion was low in the west after sunset around March and April. By June of each year, the Hunter lies behind the sun as seen from Earth. Orion only returned to visibility in Earth’s sky about a month ago. When a constellation becomes visible again, after being behind the sun, it always appears in the east before sunrise.

That’s because – as Earth moves around the sun – all stars rise two hours earlier with each passing month. So Orion is now higher at dawn than it was a month ago.

As seen from the Northern Hemisphere, Orion precedes Sirius into the sky. After Orion first appears at dawn, you can count on Sirius to appear in the morning sky a few weeks later.

Palm trees under a starry sky, including Orion and a bright dot above the trees, Sirius. — Nikunj Rawal in Gir National Park, India, captured this photo of Orion on November 21, 2020. Note the brightest star at the bottom, Sirius. Thank you, Nikunj!

The heliacal rising of Sirius

Now will be a good time to look for the heliacal rising of Sirius. In other words, what is the first date that you can see Sirius, with the unaided eye, in your morning sky? It depends on your latitude.

Below are a couple of graphs showing when you can expect to first see Sirius in your eastern predawn sky. They are designed for average eyesight, average weather, and from near sea level.

Graph with arcing line of dots, latitude on Y axis and dates across the bottom. — The heliacal rising of Sirius from latitudes 70 degrees to -20 degrees. Philadelphia, for example, is at 40 degrees north latitude, so it will see Sirius reappear in the morning sky around August 17. Based upon calculations by Culture Diff. Graph via Don Machholz.

Graph with line of dots arcing up to the right. Date on the x axis and latitude on the y axis. — The heliacal rising of Sirius from latitudes 60 degrees to 10 degrees. Based upon calculations by Culture Diff. Graph via Don Machholz.

The return of Sirius and the colors of the stars

With Sirius and the stars of Orion low in your sky, you might notice that their light shimmers in various colors. But it’s not the stars that are changing; this is the prismatic effect of Earth’s atmosphere. As seen through a greater-than-usual thickness of atmosphere in the direction of the horizon, the mostly white light of Sirius can be broken up into striking ” rel=”noopener” target=”_blank”>colors on a summer morning.

But stars can be intrinsically colorful, too. Be sure to notice the reddish color of Betelgeuse when you watch Orion rise in these late summer months.