Solstice and equinox are not the same kind of event
One is a turning point in declination, the other a crossing. The difference explains why the seasons lag, and why the Sun is closest in January.
The four season markers are two pairs of different things, and treating them as one category produces most of the confusion about seasons.
A solstice is a turning point. The Sun’s declination reaches +23.44° in June or −23.44° in December and stops moving — it is an extremum, where the rate of change is zero. A equinox is a crossing. The declination passes through zero on its way from one extreme to the other.
That distinction has a visible consequence: at the solstice, day length barely changes for a week either side. At the equinox, day length is changing fastest — by two or three minutes a day at mid-latitudes.
The 2026 instants
| Event | 2026 instant (UTC) | Declination |
|---|---|---|
| March equinox | March 20 14:46 | 0° |
| June solstice | June 21 08:25 | +23.44° |
| September equinox | September 23 00:06 | 0° |
| December solstice | December 21 20:50 | −23.44° |
These are cited from the USNO rather than computed here, and the reason is on the lunar phase page’s sibling problem: the solar method this site uses is good to about 0.01°, which is ±15 minutes of time. That is fine for a rise time and not fine for printing an equinox to the minute. When the calculation cannot reach the published precision, the published number is quoted.
Note how unequally spaced they are: 92.7 days from March to June, 93.7 to September, 89.9 to December and 89.0 back to March. Earth moves faster when it is nearer the Sun, which it is in January. The seasons are not a quarter of a year each; they are as long as the orbit makes them.
Why the seasons do not line up with distance
Earth is closest to the Sun in early January — about 147.1 million km against 152.1 million at aphelion in early July. The northern hemisphere is in winter at the near point.
The 3.4% variation in distance changes the total energy received by about 7%, which is real but small. The tilt changes the angle at which sunlight strikes the ground, which changes the energy per square meter by far more: at 40°N, the noon Sun is about 26.5° above the horizon at the December solstice and 73.5° at the June one. The same beam spread over a much larger area is the dominant effect, which is why the two hemispheres have opposite seasons at the same moment.
If distance mattered more than tilt, both hemispheres would be hot in January.
Why summer arrives late
The solstice is the moment of maximum insolation, not of maximum temperature. Ground and water absorb and re- release heat with a lag, so the warmest weeks come roughly a month later — July and August in the northern hemisphere, not late June. The lag is longer over oceans, which is why a coastal city’s hottest day comes later than an inland city’s at the same latitude.
This is also why “midsummer” means late June in some traditions and high summer in others: one marks the astronomical event, the other the weather that follows it.
The equinox is not equal
The one thing the word promises, it does not deliver: the equinox has about seven more minutes of daylight than darkness. Refraction lifts the Sun by 34′ and its disc is 16′ in radius, so it is above the horizon while geometrically below it at both ends of the day. The equinox day-length page does the arithmetic.
Season instants cited from the USNO; declinations and illumination figures are computed. The 23.44° obliquity drifts by about 0.47 arcseconds per year, which is not relevant on a human timescale but is why old almanacs disagree slightly with new ones.
Frequently asked questions
- Why is the Sun closest to Earth in January, when the northern hemisphere is coldest?
- Because distance is not what causes seasons — the tilt is. Earth reaches perihelion in early January, about 5 million km closer than average, which increases the total solar energy by roughly 7%. But the northern hemisphere is tilted away from the Sun then, so sunlight arrives at a shallow angle and is spread over more ground, and that effect is far larger. If distance drove the seasons, both hemispheres would have summer at once.
- Why do the seasons start about a month after the solstice?
- Thermal inertia. The ground and the oceans take weeks to warm up and cool down, so the hottest weather comes after the most intense sunlight rather than at the same time. The lag is roughly a month on land and longer over the ocean, which is why coastal cities have later summers than inland ones at the same latitude.
- What actually happens at the equinox?
- The Sun's apparent path crosses the celestial equator, so its declination is zero — it rises due east and sets due west everywhere on Earth. It is not a day of equal day and night; the day runs about seven minutes long, for reasons to do with refraction and the Sun's width.
- Are the four instants equally spaced?
- No. Earth moves faster in its orbit near perihelion in January, so the December-to-March quarter takes about 89 days while the June-to-September quarter takes about 93.6. The seasons are genuinely different lengths, which is a direct observation of the orbit's eccentricity.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- U.S. Naval Observatory, Astronomical Applications Department — the 2026 instants: March equinox March 20 14:46 UTC, June solstice June 21 08:25 UTC, September equinox September 23 00:06 UTC, December solstice December 21 20:50 UTC (aa.usno.navy.mil)
- NASA Science — Seasons: axial tilt of 23.44° as the cause of the seasons, and the perihelion–aphelion range of about 5 million km (science.nasa.gov)
- Meeus, Astronomical Algorithms, 2nd edition, chapter 25 — apparent solar longitude, the definition of the equinoxes and solstices, and the reason this site cites published instants rather than solving for them
- NOAA — the seasonal lag between peak insolation and peak temperature, and its dependence on land versus ocean