Solstices and Equinoxes Explained

Time & Astronomy

Four moments each year mark the corners of the calendar: two solstices, when the Sun reaches its highest or lowest point in the sky at noon, and two equinoxes, when it crosses the celestial equator. All four are consequences of a single fact — the Earth spins on a tilted axis — and all four are instants rather than days, published in UTC and therefore landing on different dates in different countries.

Key takeaways

Seasons are caused by the 23.4 degree tilt of the axis of the Earth, not by its distance from the Sun.
We are in fact closest to the Sun in early January, in the middle of the northern winter.
Each solstice and equinox is a single moment, not a whole day, and the local date depends on your time zone.
The dates drift because a year is 365.2422 days long: each event falls about six hours later annually, then jumps back on a leap year.
The equinox is not a day of exactly equal light and darkness. Refraction and the way sunrise is defined add several minutes of daylight.
The earliest sunset does not fall on the shortest day, and the latest sunrise does not either.

The Tilt, Not the Distance

The axis of the Earth leans 23.44 degrees away from vertical relative to its orbit, and it keeps pointing in the same direction all year. For half the orbit the northern hemisphere leans towards the Sun and for the other half it leans away, which changes two things at once: how high the Sun climbs at noon, and how many hours it spends above the horizon. Both feed the same result, since a high Sun delivers more energy per square metre and a long day delivers it for longer.

The persistent folk explanation — that summer is when we are closer to the Sun — is not merely imprecise, it is backwards for half the planet. The Earth reaches perihelion, its closest approach, in early January, at about 147.1 million kilometres, and aphelion in early July at about 152.1 million. Northern winters happen at the near point. The three per cent difference in distance is simply swamped by the tilt.

Because the tilt works in opposite directions either side of the equator, the seasons are mirrored. The June solstice is midsummer in London and midwinter in Sydney and Cape Town. Near the equator, in Quito or Singapore, the whole apparatus barely registers: day length varies by only a few minutes across the year, and the seasons that matter are wet and dry rather than warm and cold.

The Four Moments

Event	Usual date (UTC)	What happens	In the north
March equinox	19–21 March	The Sun crosses the equator going north	Start of astronomical spring
June solstice	20–21 June	The Sun reaches its northernmost point	Longest day, start of summer
September equinox	22–23 September	The Sun crosses the equator going south	Start of astronomical autumn
December solstice	21–22 December	The Sun reaches its southernmost point	Shortest day, start of winter

The words describe the motion precisely. Solstice comes from the Latin for the Sun standing still: for several days around it the noon Sun stops climbing or descending and hovers at nearly the same height, which is exactly what an object looks like at the turning point of a swing. Equinox means equal night, a claim examined below and found slightly wanting.

Note that these are astronomical seasons. Meteorologists prefer whole months — spring beginning on 1 March, summer on 1 June and so on — because complete months make climate statistics comparable from year to year. Neither definition is more correct; they answer different questions.

Why the Dates Move

A tropical year is 365.2422 days, not 365. Each event therefore arrives about 5 hours 49 minutes later in clock terms than the year before, drifting forward until a leap day pulls it back by almost a full 24 hours. Over a four-year cycle the June solstice can move between 20 and 21 June, and the December solstice between 21 and 22 December.

The Gregorian calendar exists to keep this drift bounded. Skipping three leap days every four centuries — 1700, 1800 and 1900 were not leap years, 2000 was — keeps the equinoxes anchored near their traditional dates rather than sliding through the calendar as they did under the Julian system. It is the same class of problem as the leap second in modern timekeeping: a natural period that refuses to divide neatly into a convenient unit, discussed further in the history of standard time.

A Single Instant, and Therefore Two Dates

A solstice is not a day. It is the exact moment the centre of the Sun reaches its greatest declination, and it is published in UTC like every other astronomical event. Convert that instant to local time and the calendar date can change:

City	A solstice at 22:30 UTC on 21 December falls at
New York (UTC−5)	17:30 on 21 December
London (UTC+0)	22:30 on 21 December
Madrid (UTC+1)	23:30 on 21 December
Tokyo (UTC+9)	07:30 on 22 December
Auckland (UTC+13 in summer)	11:30 on 22 December

This is why almanacs in different countries print different dates for the same solstice, and why the answer to “when does summer start?” genuinely depends on where you ask. It is not a discrepancy but a conversion, of exactly the sort set out in How Astronomical Events Are Converted to Local Time. The same trap applies to the instant of a full moon and to the phases of a lunar eclipse.

The Equinox Is Not a Day of Equal Light

On the equinox the Sun is above the horizon for about 12 hours and 7 minutes at the equator, and a few minutes longer than that at higher latitudes. Two definitions conspire to add the extra:

Sunrise counts the upper edge, not the centre. The Sun is officially up when its first sliver clears the horizon, half a diameter before its midpoint would.
The atmosphere bends light. Refraction lifts the image of the Sun by roughly the width of the disc, so you see it rise before it geometrically arrives and set after it has geometrically gone.

The day of genuinely equal light and darkness has its own name, the equilux, and it falls a few days before the March equinox in the northern hemisphere and a few days after the September one. How many days depends on latitude: close to the equator it never quite happens at all. The mechanics of that horizon are covered in Sunrise, Sunset and Twilight Explained.

How Much Daylight the Solstice Actually Delivers

Approximate daylight at the June solstice, by latitude:

Latitude	Roughly	Daylight on the June solstice
0°	Quito, Singapore	About 12 hours
40°N	New York, Madrid	About 15 hours
51°N	London	About 16 hours 40 minutes
64°N	Reykjavik	About 21 hours, with no real darkness
Above 66.6°N	Tromso	24 hours: the midnight sun

Past the polar circles the arithmetic breaks down entirely and the Sun simply stops setting, or stops rising, for weeks or months at a time — taken to its logical conclusion at the poles themselves, where there is one sunrise and one sunset a year, as described in What Time Is It at the North and South Pole?

The Shortest Day Is Not the Darkest Evening

A reliable surprise of midwinter: the earliest sunset comes before the December solstice and the latest sunrise comes after it. At around 40 degrees of latitude the gap is a fortnight in each direction, so evenings begin drawing out well before the shortest day while mornings continue getting darker into January.

The cause is that a solar day is not exactly 24 hours. The Earth travels faster along its orbit near perihelion and its axis is tilted, so true solar noon slides back and forth against clock noon through the year by as much as 16 minutes — the equation of time. Daylight length is symmetrical about the solstice, but the whole day is shifted slightly, and the two effects add on one side and cancel on the other.

That same mismatch between the Sun and the clock is the reason mean time replaced solar time in the first place, and the reason daylight saving is keyed to fixed calendar dates instead of to the solstices themselves. Clock changes cluster near the equinoxes because that is where daylight is changing fastest, not because of any astronomical rule; the actual dates are set by legislation, as listed in When Do the Clocks Change?

Frequently Asked Questions

What is the difference between a solstice and an equinox?

A solstice is the moment the Sun reaches its furthest point north or south, producing the longest and shortest days. An equinox is the moment it crosses the celestial equator, when day and night are close to equal everywhere on Earth.

Why do solstice dates change from year to year?

Because a year is 365.2422 days rather than a whole number. Each solstice occurs about six hours later than the previous one, then jumps back nearly a day when a leap day is inserted, so the date oscillates across a two-day window.

Are day and night exactly equal on the equinox?

Not exactly. Daylight runs several minutes longer, because sunrise is measured from the upper edge of the Sun and the atmosphere refracts its image upwards. The day of truly equal light, the equilux, falls a few days before the March equinox in the northern hemisphere.

Is the Earth closer to the Sun in summer?

No. The Earth is closest to the Sun in early January and furthest in early July. Seasons come from the 23.4 degree tilt of the axis, which changes how high the Sun climbs and how long it stays up, not from the small variation in distance.

Why does my country list a different solstice date?

Because the solstice is a single instant published in UTC. Converted to local time it can fall on the previous or the following calendar date, so a solstice late on 21 December in Europe is already 22 December in Japan and New Zealand.