Then we’ll explore some ancient traditions and celebrations around the world. How this all works has fascinated people for thousands of years.įirst we’ll look at the science and precise timing behind the solstice. Winter solstice 2022, the shortest day of year and the official first day of winter, is on Wednesday, December 21 (well, for a decent chunk of the world anyway).
So, the exact dates can shift by a day, even though the 21st of each month is usually correct.For the past six months, the days have grown shorter and the nights have grown longer in the Northern Hemisphere. I say "about" for the dates because the Earth's orbit and the calendar we use are not exactly synchronized (if they were, we wouldn't need leap years). Vernal Equinox: official start of spring (about March 21, top of figure).Winter Solstice: official start of winter (about December 21, right of figure).Autumnal Equinox: official start of fall (about September 21, bottom of figure).Summer Solstice: official start of summer (about June 21, left of figure).So, these points in the Sun's motion define the seasons: The two days exactly in between the solstices, when the amount of daylight and darkness are equal, are known as the equinoxes. The shortest day of the year, when the Sun rises at its southernmost point, is called the Winter Solstice. The longest day of the year, when the Sun rises at its northernmost point, is called the Summer Solstice. Halfway between these points (top and bottom of figure), the Sun is positioned directly perpendicular to the Earth's axis, which means all of the Earth is in sunlight for 12 hours, and in darkness for 12 hours.įrom the viewpoint of someone on the Earth, this means that the rising/setting point of the Sun appears to move along the horizon during the course of a year, which makes its daily path through the sky longer or shorter. Likewise, when the Earth is on the opposite side of its orbit, with its axis pointing away from the Sun, then people in the Northern Hemisphere are exposed to the Sun for the minimum amount of time, and we have winter.Ĭhallenge - Can you deduce why it is that the seasons in the Southern Hemisphere are exactly opposite to those in the Northern? When the Earth is at the point in its orbit where its axis is pointing at the Sun (far left), then people in the Northern Hemisphere are exposed to the Sun for the maximum amount of time. Note that the Earth's axis always points in the same direction: it does not rotate or shift as the Earth orbits the Sun. The axis of the Earth's spin is canted at a 23° angle with respect to the plane of the Earth's orbit around the Sun, which therefore means that the Sun's position in the sky depends very much on where the Earth is in its orbit.Ĭonsider this schematic of the Earth's orbit. Throughout the course of a year, the Sun's position on the horizon as it rises and sets each day slowly shifts north and south. The answer is that the traditional boundaries between the four seasons are related to the motion of the Sun, and the tradition dates from antiquity. Phyx 103-0, The Seasons The Official SeasonsĮver wonder what the weatherperson means when they say that such-and-such day is the "official" start of one of the four seasons? What makes it official, and who decided?
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