Equinox and Solstice Calculator

In short An equinox or solstice is an instant, not a day. It is defined purely by the sun apparent longitude: 0 degrees at the March equinox, 90 at the June solstice, 180 in September and 270 in December. Everything people associate with them — equal day and night, the longest day — is a consequence, and not always an exact one. Because they are instants, which calendar date they land on depends on your time zone, and that is why the March equinox is sometimes the 20th and sometimes the 21st. The 2026 March equinox falls at 14:46 UT on 20 March. The seasons are not equal in length, and the reason is the shape of the earth orbit. In 2026 northern spring runs 92.74 days, summer 93.65, autumn 89.86 and winter 88.98 — a spread of nearly five days between the longest and shortest. The earth is near aphelion, its furthest point from the sun, during northern summer, and by Kepler second law it moves slowest when it is furthest away, so it takes longer to cover that 90 degrees of orbit. Northern winter is the shortest season because the earth is near perihelion in early January and moving at its fastest. Two things worth knowing that follow from all this. The equinox day is not twelve hours long — it runs several minutes over, everywhere, because sunrise is measured on the sun upper limb and includes refraction. And the earliest sunset does not fall on the shortest day: the equation of time shifts solar noon through December, so the earliest sunset comes a week or two before the solstice and the latest sunrise a week or two after it.