Traditionally, we’ve celebrated the beginning of spring on March 21, but astronomers and calendar manufacturers alike now say that the spring season starts one day earlier, March 20, in all time zones in North America.
How could the first day of spring change from year to year?
There are a few reasons why seasonal dates can vary from year to year. The first is that a year is not an even number of days and neither are the seasons.
Another reason is that the earth’s elliptical orbit is changing its orientation (skew), which causes the earth’s axis to constantly point in a different direction, called precession. Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time the earth reaches each 90-degree location in its orbit around the sun.
The pull of gravity from the other planets also affects the location of the earth in its orbit. The current seasonal lengths for the Northern Hemisphere are:
Summer — 93.641 days
Autumn — 89.834 days
Winter — 88.994 days
Spring — 92.771 days
As you can see, the warm seasons, spring and summer, combined are 7.584 days longer than the colder seasons, fall and winter (good news for warm weather admirers). However, spring is currently being reduced by approximately one minute per year and winter by about one-half a minute per year. Summer is gaining the minute lost from spring, and autumn is gaining the half a minute lost from winter. Winter is the shortest astronomical season, and with its seasonal duration continuing to decrease, it is expected to attain its minimum value — 88.71 days — by about the year 3500.
Length of Day Vs. Night
Another complication revolving around the vernal equinox concerns the length of day versus night. We have been taught that on the first days of spring and autumn, the day and night are equal to exactly 12 hours all over the world. Yet, if you check the calendar pages in our Almanac, you will find that this is not so. In fact, our tables tell you that on the days of the spring and fall equinox, the length of daylight is actually longer than darkness by several minutes.
The reason this happens can be attributed to our atmosphere. If the earth was a planet that did not have an atmosphere, then yes, on the equinox days the length of the day and night would be exactly even.
However, our atmosphere acts like a lens and refracts (bends) its light above the edge of the horizon. Put in another way, when you watch the sun either coming up above the horizon at sunrise or going down below the horizon at sunset, you are looking at an illusion – the sun is not really there, but already below the horizon.
As a result, we actually end up seeing the sun for a few minutes before its disc actually rises and for a few minutes after it has actually set. Thus, thanks to atmospheric refraction, the length of daylight on any given day is increased by approximately six or seven minutes.