The dog days or dog days of summer are the hot, sultry days of summer. They were historically the period following the heliacal rising of the star system Sirius, which Hellenistic astrology connected with heat, drought, sudden thunderstorms, lethargy, fever, mad dogs, and bad luck. They are now taken to be the hottest, most uncomfortable part of summer in the Northern Hemisphere.
Trees and grass on Crete dried out by the August heat
A 9th-century astronomical manuscript, including an illustration of the constellation "Sirius"
"Some Popular Alleviations of the Dog Days in Hotter New-York" in 1904, including children piled into a public fountain "when the 'cop' is not looking".
Orion (right) and Sirius (bottom), as seen from the Hubble Space Telescope
The heliacal rising of a star or a planet occurs annually when it first becomes visible above the eastern horizon at dawn just before sunrise after a complete orbit of the Earth around the Sun. Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation, as well as the farming season. Helical rising is one of several types of risings and settings, mostly they are grouped into morning and evening risings and settings of objects in the sky. Culmination in the evening and then morning is set apart by half a year, while on the other hand risings and settings in the evenings and the mornings are only at the equator set apart by half a year.
Sirius is the fixed star with the greatest apparent magnitude and one which is almost non-variable. The Pleiades, a key feature of Taurus shown across Orion in the same photograph also experience an annual period of visibility ("rising and setting"). Photo taken at sunset.
