The origin of the 'dog days of summer'

Sirius and the dog days

Photo: Contributed

A map of constellations.

Around this time of year, if the weather is clear, just before dawn you might be able to see Sirius, the brightest looking star in the sky after the Sun.

Sirius, also known as the "Dog Star", lies in constellation of Canis Major, "The Great Dog". It's appearance in the pre-dawn sky marks the "Dog Days" of summer, and to the ancient Egyptians, marked the beginning of the annual flooding of the Nile, upon which the Egyptian nation depended. Apparently this time was important to dog owners, because of the possibility their pets might go mad.

Our ancestors did not have accurate clocks or calendars, but managed the timing of their lives using the rhythms of the sky. The rotation of the Earth, the time between when the Sun lay in the south to the next time it did gave us our day, which we divided into 24 hours.

The time between two consecutive Full Moons gave us our first version of the month (mo(o)nth). The points on the horizon where the Sun rose and set gave us clear markers for the seasons. When the sunrise and sunset points were at their northernmost extremes on the horizon, and we had the day with the maximum number of hours of daylight, we designated that the summer solstice.

On the other hand, when the Sun rose and set at its southernmost points on the horizon we had the day with the smallest number of hours of daylight, which we named the winter solstice. Halfway between these extremes are the points, lying due East and West, where the number of hours of daylight and darkness are equal. These are the spring and autumn equinoxes.

To see how our ancestors did this, just take note of the points where the Sun rises and sets over the year. It is very obvious. The time between two consecutive winter or summer solstices, or two consecutive spring or autumn equinoxes gives us the year. Using this system did not require our ancestors to know what exactly is going on up there. They just needed to watch.

Having this astronomical calendar was not just of academic interest. Lives could depend on it. Many peoples depended on knowing when animals are migrating or fishes moving up or down the rivers. When agriculture became an important activity, getting those crops in the ground early enough to mature, but late enough to avoid being killed by frost or drought was (and still is) critical. Knowing when your farm was about to be flooded by the Nile was important too. This flooding was not a bad thing; it was a good thing because it deposited silt on the land, refreshing its fertility.

Managing our lives by watching the sky worked for many centuries. However, when it became necessary to precisely define when various religious celebrations were to be held, on the same day over whole countries and beyond, things got more complicated.

In some churches, such as Saint-Sulpice, in Paris, a hole was made in the south-facing wall so that the Noon Sun cast a beam of light on a long scale, laid out across the floor and partway up the opposite wall. As the Sun got higher and lower in the sky over the year, the spot of light fell on the scale, marking the times for the many religious events. That solved the problem for that church and maybe those nearby, but it did not solve the event coordination problem.

To really address the problem a proper calendar was needed. This ran into the problem that the year is not a whole number of days, so to keep a workable calendar in step with the seasons requires changing the lengths of the months and adding leap days and seconds as needed to keep it locked to the sky. There is another problem. Our planet's tidal interaction with the Moon is sending the Moon further away and slowing our planet's rotation, making the day and the mo(o)nth longer.

Saturn rises around midnight and Venus around 2am. Jupiter is low in the east before dawn. Venus is brighter and whiter. Mars is extremely low in the west after sunset, getting lost in the glow. The Moon will reach First Quarter on the 1st.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.