Spring begins on March 20 with the vernal equinox, a day of nearly equal daylight and darkness. There’s another equinox in September that marks the start of autumn. And between the equinoxes we have the summer and winter solstices, the longest and shortest days of the year, in June and December.
Our changing seasons and these four cardinal points of our calendar year are astronomical in origin. Earth is tilted with respect to its rotation by about 23 degrees. This means that instead of our daily 24-hour rotation being nicely perpendicular to our 365-day orbit around the Sun, we’re a little skewed. The effect of this is that our northern and southern hemispheres are periodically tilted toward the Sun or away from it.
When the northern hemisphere is tilted toward the Sun it receives more direct sunlight. The atmosphere and oceans warm up, and we call it summer. When the northern hemisphere is tilted away, the sunlight we receive is more indirect and we receive less of it. Think of how the Sun is noticeably lower in the sky in December and January. Things cool down, and we call it winter. (Of course, I’m writing as a North American; it’s the opposite in the southern hemisphere. When we have winter, they have summer, and vice versa.)
Without this tilt to our rotation, we would have no seasons. The weather would remain generally the same in a given latitude throughout the year. But thanks to Earth’s tilt, you can stay put on the Outer Cape all year and experience warm beach days, crisp autumn afternoons, howling snowstorms, and bright spring mornings.
Many cultures past and present celebrate holidays in synchronization with these axes of the solar year. For the people of pre-Christian Europe, midsummer — the summer solstice — was one of the most important religious events of the year. Midsummer remains an important national holiday in countries like Sweden, Finland, and Estonia.
Rosh Hashanah, the Jewish New Year, is linked to the lunisolar calendar, which combines both the Moon phase and the time of the solar year. So, its date shifts around to some degree. But it always occurs in September or October, and often very close to (if not actually on) the autumnal equinox.
The winter solstice and longest night inspire celebrations — festivals of light to fend off the cold and darkness.
With the vernal equinox come holidays that celebrate the end of winter and the rebirth of life around us. It is the first day of the Iranian New Year, or Nowruz. This tradition dates to ancient Persia and Zoroastrianism. In Japan, the vernal equinox is a national holiday called Shunbun no hi. It’s a time for family gatherings, visiting the graves of ancestors, and seeing the cherry trees in bloom. And there is Easter. Its exact date changes from year to year but is linked to the vernal equinox. It falls on the first Sunday after the full Moon closest to the equinox.
The photo accompanying this story is a composite of three time-lapse photos taken from the same location at the summer solstice, autumnal equinox, and winter solstice. You can see the Sun rising from a different spot on the horizon in each. But the Sun hasn’t really moved; we have. The photo shows how, as Earth’s axis tilts toward the Sun at the summer solstice and then away from it in winter, the Sun’s location at the horizon and in the sky shift.
If you’re up for a fun astronomical challenge, note the position of the Sun at the horizon when it rises or sets. Take a photo — being careful not to damage your camera or your eyes. Then return to that same location at the summer solstice, autumnal equinox, and winter solstice, each time noting the Sun’s location at the horizon. You’ll see the shift the tilt causes for yourself.
Maybe you’ll be inspired to raise a stone monument in alignment with the Sun on one of these dates. That’s what they did around 2,500 BCE at Stonehenge, one of the most famous prehistoric structures that mark the moving Sun. The stones there align with sunrise on the summer solstice and sunset on the winter solstice. The ancient Britons left no written records, so we can only guess at the purpose of the monument. But we know the construction of Stonehenge spanned generations and required the coordinated efforts of thousands of people. The solar alignments are not by chance.
Whatever your traditions or beliefs, I hope you’ll celebrate the vernal equinox. You don’t have to observe the perfectly aligned rising Sun at Stonehenge as the ancient Britons did. Just step outside to watch the Sun rise or set. Clear skies!