At 9.01am today, March 20th, the sun crossed the Earth’s equator, marking the vernal equinox, the date when the direction of sunrise is due east and sunset due west.
The Earth’s axis is at right angles to the line from sun to Earth, the sun is directly over the equator and the lengths of day and night are approximately equal. Equinoxes are of interest for many reasons of science, history, religion, agriculture and technology.
To be precise, the time from sunrise to sunset on the equinox is slightly more than 12 hours. One reason is that, due to bending of light by the atmosphere, we can see the sun while it is still below the horizon, making sunrise earlier and sunset later than simple geometry might suggest.
Another reason is the size of the solar disk, which is not just a point source of light. The spring date on which day and night are of equal length is a few days before the equinox and is known as the vernal equilux.
The sun follows a path called the ecliptic, which crosses the plane of the celestial equator twice each year, around March 20th and September 23rd. The ecliptic is the apparent orbit of the sun through the zodiac over a year and the celestial equator is an extension of the Earth’s equator into space.
The vernal equinox is often taken to mark the beginning of spring and the rebirth of nature. This is especially striking in higher latitudes: at the North Pole today, the sun rises for the first time in six months and, for the next few days it will skim westward along the horizon, making a complete circuit every day. For six months, it will never set in the Arctic region.
Precession of the equinoxes
At the spring equinox, the sun returns to a particular spot in the zodiac. The Greek astronomer Hipparchus called this the “first point of Aries” as, in his day, it was in the constellation of Aries. However, he was a meticulous observer and noticed a gradual change of position of about one degree every 72 years, implying a full cycle in 26,000 years. The equinoctial point is now in Pisces.
Newton showed how the Earth’s oblate shape was due to its rotation, and he explained that the gradual change in the time of the equinoxes is due to the gravitational pull of the moon and sun on the equatorial bulge, causing the planet to precess like a giant spinning top as its axis of rotation slowly traces out a circle in the sky.
We are fortunate to have a large moon, which has a stabilising effect on the Earth’s axis. Simulations of the solar system show that, in the absence of the moon, the Earth’s tilt would vary in an irregular manner, and the precise times of the equinox would be unpredictable. More importantly, the seasonal cycle would become chaotic, with disastrous global consequences.
Due to tidal effects, the moon is moving slowly away from Earth, although its departure is so gradual that no such catastrophe is expected soon. Ultimately, we may expect chaotic swings in climate, but not for about three or four billion years. However, don’t worry: the increasing luminosity of the sun will vaporise the oceans long before that.
Peter Lynch is emeritus professor at UCD School of Mathematics & Statistics – he blogs at thatsmaths.com
