One of the most important events in the daily life of ancient Egypt was the annual flooding of the Nile. During this period, the nourishing river would overflow its banks, sweep into arable lands and deposit fertile silt ideal for farming. To predict when the flood would occur, Egyptian priests relied on a celestial event: the heliacal rising of the star Sirius.
What is the heliacal rising of a star?
Given how Earth moves around the Sun, we have the impression that our star is drifting across the sky. When the Sun is found in a certain region of the sky, its brightness prevents us from seeing any stars in its vicinity. As the days go by, the Sun changes position, allowing the stars it was concealing to be visible again. The heliacal rising of a star is the first day when this star becomes visible again in the east in the light of dawn just before sunrise.
The ancient Egyptians noticed that the heliacal rising of the star Sirius, the brightest in the night sky, would occur a short time before the annual flooding of the Nile. The heliacal rising of Sirius therefore kicked off the farming season in ancient Egypt.
Egyptian priests also used the rising of other stars to mark the passage of time during the night. These “star clocks” enabled them to divide the night into twelve hours.
In the time of the pharaohs, the heliacal rising of Sirius occurred in early July. In modern times, the precession of the equinoxes has pushed the phenomenon back to early August, meaning it no longer coincides with the flooding of the Nile. In any case, the famous Aswan High Dam, built in the 1960s, has since prevented the annual flooding of the Nile.
The planets in July
This month, we have a chance to observe all the planets visible to the naked eye. The Moon will guide us toward each of them in turn.
Let’s start our planetary tour with Mars. The summer of 2018 is an excellent time for observing the red planet. Mars is in opposition on July 27 and thus visible the entire night, from sunset to sunrise. On July 31, the distance between Earth and Mars is the shortest it’s been since 2003, only 57.6 million kilometres.
On the night of June 30 to July 1, the Moon is 5 degrees to the upper right of Mars. They’ll be at their best in the second half of the night. On July 26, the Moon is found once again near Mars. It is almost full and lies to the upper right of the red planet. The next night, the full Moon now shines to the left of Mars.
Mercury and Venus in the evening sky
Planet watchers can take on the challenge of spotting Mercury at twilight in the first half of July. It first shows up about 45 minutes after sunset very low on the west-northwestern horizon, but the planet is observable only for about 15 minutes before it also sets. On July 14, a thin crescent Moon lies less than 2 degrees above Mercury. The next night, the crescent Moon is seen just to the right of dazzling Venus.
Jupiter and Saturn
On the evening of July 20, the Moon now guides us toward Jupiter. The first quarter Moon is found just over 3 degrees to the upper right of the Jovian planet. The two celestial bodies set shortly after midnight.
We wrap up our tour of the planets with Saturn on the night of July 24 to 25. In the early evening, the Moon shines 2 ½ degrees above the ringed planet, but it draws closer to Saturn as the night wears on. When the two celestial bodies set around 3:30 a.m., they’re just over 1 degree apart.
Finally, keep in mind that Earth is in aphelion (the point in its orbit farthest from the Sun) on July 6 at 1 p.m. At that time, we’ll be 152,095,566 kilometres from our star.