The "dog days of summer" is a term for the hot and humid weather some parts of the world experience during the summer months. This occurs soon after the Summer Solstice, in late June, at the beginning of the worst of summer's heat. But why do we call it the "Dog Days"?

The Nile River and Ancient Egypt

The Nile River flooded each year in ancient Egypt, beginning in late June. The people welcomed this event, called the Inundation, when the floodwaters brought rich soil needed to grow crops in what was otherwise a desert. No one in Egypt knew exactly when the flooding would start, but people noticed the water began to rise on the days when the star Sopdet began to rise before the sun. Sopdet (or Sirius as we more commonly know it) and the Inundation became so important to the Egyptians' survival that they started their new year with the new moon that followed the star's first appearance on the eastern horizon.

Ancient Greece and Rome

In ancient Greece and Rome, the Dog Days were a time of drought, bad luck, and unrest. The extreme heat was believed to drive dogs and men mad.

The Significance of Sirius

Three ancient civilizations—Greece, Egypt, and Rome—believed that the dawn rising of Sirius in mid-to-late summer contributed to the extremely hot weather. In other words, they believed super-bright Sirius and our sun's combined heat caused summer's soaring temperatures. Even the name "Sirius" refers to heat. It stems from the Ancient Greek seírios, meaning "scorching." Although we know that is not the case today, the name stuck.

The Origin of 'Dog Days'

This period of hot summer weather comes at the same time as the heliacal ("at sunrise") rise of the "dog star" - Sirius. Sirius is part of the constellation Canis Majoris (Alpha Canis Majoris)—the "Greater Dog"—where Sirius gets its 'Dog Star' nickname. This would be one of Orion's two hunting dogs in constellation lore, and all three constellations can be seen together in the summer sky.

The Brightness of Sirius

Sirius is the brightest star in the night sky. Under the right conditions, it can be seen with the naked eye during the day. A binary star system, Sirius has two white stars orbiting each other, separated by roughly the distance between the Sun and Uranus. Sirius's companion star is the Pup. Though the name signifies youth, Pup is a dead star called a white dwarf. Once a mighty star, the Pup today is Earth-sized and too faint to see without a telescope.

The Flickering of Sirius

Due to its flickering brightness and apparent color changes, sometimes people call this blue-white star a rainbow star, and a few people even report Sirius as a UFO. These changes happen when such a bright star as Sirius shines through the blanket of Earth's atmosphere. Earth's air's varying density and temperature affect starlight, especially when we see the star low in the sky.

So... rainbow, UFO, dog, or however you view the star Sirius, get out, enjoy the warm summer nights, and check out the Dog Star for yourself.