Planetarium

"I read an article that said that winter is the shortest season. Is that right? It feels like it is never going to end!"



 - Stowe, VT

Greetings from glaciation central here in Portland, Maine.

Yes, this winter has proven so cold, brutal and snow-laden that one retains only the vaguest memories of warmer weather. As difficult as it is to believe, astronomical winter is, indeed, the shortest of the four seasons. Or, more correctly, Northern Hemisphere winter, also known as Southern Hemisphere summer, is the shortest season.

First, we point out that seasons have different definitions:

Meteorologically, the seasons are defined as follows:
Winter starts on December 1 and ends on February 28/29.
Spring begins on March 1 and ends on May 30.
Summer starts on June 1 and ends on August 31.
Autumn begins on September 1 and ends on November 30.

Astronomically, the dates are different:
Winter begins on the winter (December) solstice, which generally occurs on or around December 21st.
Spring begins on the vernal (March) equinox, which happens on the 20 or 21st of March.
Summer starts on the summer (June) solstice, around June 20 or 21st.
Autumn begins on the autumnal (September) equinox, which can occur on the 22, 23 or even the 24th of September.*

We have seasons because Earth is tilted by about 23.5 degrees relative to the vertical. As Earth revolves around the Sun, the Northern Hemisphere is alternately oriented toward and then away from the Sun. Consequently, the Sun's altitude varies with the changing orientation: higher in the spring and summer; lower in the autumn and winter. During the spring and summer, the Sun remains above the horizon for more than twelve hours and its higher angle enables it to heat us more efficiently than it does in the autumn and winter.
Earth orbital graphic

If Earth's orbit were perfectly circular, our planet would require the same amount of time to progress from one seasonal point to another.  However, the orbit is elliptical, so Earth's distance from the Sun varies continuously and so, too, does its orbital speed. (The closer a planet moves toward the Sun, the faster its orbital velocity will be.) Earth is closest to the Sun (perihelion) between January 1-4 and at its greatest distance (aphelion) around July 2-4. As Earth is closet to the Sun during winter, it is moving fastest along its orbit. In the summer, Earth is farther away and therefore its orbital speed is slower. Consequently, the time our planet requires to proceed from the winter solstice point to the vernal equinox is almost 89 days (88.9). The time that elapses between the summer solstice and autumnal equinox is 93.6 days. Our summer is more than four days longer than our summer.  

The season durations:
Winter     88.9 days
Spring     92.8 days
Summer  93.6 days
Autumn   89.85 days

The seasonal duration values will change with time, as the perihelion date moves to later in the year over the centuries. (In the 13th century, Earth reached perihelion precisely on the December solstice.)

Even though the snow is piled eleven feet high and we'd have to give the temperature a negative handicap to push it above freezing, we can at least take solace from the assurance that winter is the shortest season.


*The last time the autumnal equinox occurred was on September 24 was 1931. The next occurrence will be 2303.