When does spring start this year? It’s complicated and depends on who you ask. This year, spring officially begins on March 19, the date of the March equinox. Equinox means equal night. But surprisingly, the equinox is not the date when all locations on Earth receive exactly twelve hours of light. On March 19, we will all have roughly twelve hours of light. But only roughly. This is partly due to a process called refraction. In any sunset, the sun has already set when we see the sun just touching the horizon. The atmosphere bends the sun’s light allowing us to see the sun when it is just below our horizon. At the South Pole, the sun is still visible all day on the March equinox. The sun skirts the horizon just above and below it on that day. But because of refraction, the sun is still visible all day even though it is not actually above the horizon for part of the day. The Northern hemisphere is also gaining light while the southern hemisphere is losing light. The change in day length is more pronounced at places farther from the equator. So if the equinox is not the date of equal nights, what is it? The equinox is the time when the sun appears directly overhead from the equator. Because of this, equinoxes not only happen at specific dates but even at specific times. This year’s equinox will happen at precisely 11:06 PM EDT on March 19.
There are, however, other ways of measuring spring. In 1780, the Societas Meteorologica Palatina defined the seasons as beginning on March 1, June 1, September 1, and December 1. Meteorologists (and other people who like to think of seasons as spanning three complete months) still use these seasons. But some people group the seasons by three whole months without using the month groups defined by the SMP. In Ireland, spring begins on February 1.
The Irish definition uses seasons based on days called cross-quarter days. Quarter days are an old name for the equinoxes and solstices. A cross-quarter day falls between these days. In the Middle Ages, these days each had a religious feast day. The Quarter Day feast days were Christmas, the Annunciation, the Feast of St. John the Baptist, and the Feast of St. Michael. These feasts have fallen somewhat out of line with the solar year over time. The cross-quarter days were Candlemas (February 2), May Day (May 1), Lammas (August 1), and All Saints Day (November 1). (In the Middle Ages, May Day and Lammas were both religious celebrations). Again these dates have shifted somewhat so the true cross-quarter days should be February 4, May 4, August 6, and November 6. So what do these days mark? Between February 4 and May 4, the northern hemisphere is gaining the most amount of light. This is spring. From May 4 to August 6, the northern hemisphere receives the most light of the year. Between August 6 and November 6, the light is decreasing at the most rapid rate while November 6 to February 4 is the darkest time of the year.
So if you want to know when spring begins this year, you can use a variety of dates. If you’re tired of winter, you can think that it has been spring for about a month. But if you aren’t tired of winter yet, you can have a few more weeks of it.