Changing of the seasons
|Equinox||September 23, 2023||2:49 EDT|
|Solstice||December 21, 2023||22:27 EST|
|Duration of Autumn: 89 days 20 hours 38 minutes|
Table of Equinoxes and Solstices
IMPORTANT NOTE: All times below given as Eastern Standard Time (EST = UT − 5 hours).
Add one hour when Daylight Savings Time (EDT) is in effect.
For years outside those given in this table, refer to the Data Services provided by the Astronomical Applications Department of the U.S. Naval Observatory.
You'll have the opportunity to choose your time zone and automatically take into account the change to Daylight Saving Time.
The cycle of the seasons
The rotational axis of the Earth is tilted 23.5 degrees relative to its orbital plane, and the orientation of that axis is pretty much fixed in space. Thus, depending on the position of the Earth along its orbit – that is, depending on the time of year – the Sun shines down more on one or the other of our planet’s hemispheres.
At a given spot on Earth, the Sun therefore appears to us to be higher or lower in the sky at midday. That annual variation in the height of the Sun is what lies behind the cycle of seasons.
The solstice is one of the two moments in the year when the Sun is found at its most northerly or southerly position in the sky.
Twice a year, the Sun slows down on its way northward or southward before finally reaching its greatest declination. At that moment it seems to pause before gradually setting off in the other direction. That’s where the word solstice originates: in Latin, solstitium, from sol (the Sun) and sistere (to stand still).
At the moment of the solstices, the length of the day (the interval between sunrise and sunset) is longest (in June, in the northern hemisphere) or shortest (in December).
The equinox occurs between two solstices, at the moment when the Sun crosses the celestial equator, meaning the moment when it’s directly above the Earth’s equator.
The name equinox also comes from the Latin: æquinoctium from æquus (equal) and nox (genitive noctis) (night), meaning “equal nighttime” (i.e., equal to daytime). At that moment, the dividing line between the day side and the night side passes over the two poles, in theory lighting up the two hemispheres equally. Night and day should therefore be of equal duration everywhere on Earth.
In reality, refraction of light rays caused by the atmosphere “lifts” the disc of the Sun in the sky: it still appears to be above the horizon when it has geometrically “set.”
At the latitude of Montréal (45.5 degrees north), atmospheric refraction lengthens the duration of daytime by about ten minutes, and the dates of the true “equiluxes” fall roughly three days before the March equinox and three days after the September equinox.
Simultaneously all over the planet
The equinoxes and solstices take place at the same moment everywhere on Earth. The date and time depend simply on the time zone used.
In the above table, the instant of equinoxes and solstices is expressed in Eastern Standard Time. Also to be observed is that, contrary to popular belief, the dates do not necessarily fall on the 21st day of the month concerned.
Astronomical seasons and climatological seasons
The moments of the equinoxes and solstices mark the start of the astronomical seasons.
Hence, in the northern hemisphere, the March equinox is the beginning of spring; the June solstice, summer; the September equinox, autumn; and the December solstice, winter. In the southern hemisphere, the seasons are opposite to those in the northern hemisphere. For example, in Australia, autumn begins with the March equinox. Note that in certain countries or cultures, equinoxes and solstices are considered to mark the middle of seasons instead.
Owing to the thermal inertia of the atmosphere, of bodies of water and of the ground, the climatological seasons lag a few weeks behind the astronomical seasons. In Montréal, for example, we find that the extreme heat of summer does not happen at the moment of the June solstice, but later in July, on average.
The same applies to the extreme cold of winter: average temperatures are at their lowest during the second half of January, roughly one month after the December solstice.
Duration of the astronomical seasons
As the interval of time between successive equinoxes and solstices is not equal, the seasons are not all of the same duration.
Because of its slightly elliptical orbit, the Earth doesn’t move at a uniform speed around the Sun. It moves more rapidly in early January, when it’s closer to the Sun, which means that we spend less time “in winter” (about 88 days, 23 hours and 37 minutes, currently).
In contrast, the Earth is further from the Sun and moves more slowly in early July, and it’s the northern summer that’s the longest of the seasons (about 93 days, 15 hours and 50 minutes).
The phenomenon of the precession of the equinoxes and the long-term variations in the shape of the Earth’s orbit mean that these durations change gradually over centuries and millennia. For instance, the northern summer is currently the longest of the seasons, and its duration will continue to lengthen until the 37th century, at that point reaching close to 94 days. It will then embark on a slow decrease, but will nevertheless remain the longest season until the year 6430, when autumn will snatch that title away.