Seasons 2021: spring begins 20 March at 09:37 UTC
Astronomical spring in the northern hemisphere opens at the March equinox, Saturday, 20 March 2021 at 09:37 UTC. Summer follows at the June solstice on 21 June 2021, autumn at the September equinox on 22 September 2021 and winter at the December solstice on 21 December 2021. South of the equator the same four instants open autumn, winter, spring and summer instead.
When does spring start in 2021?
Astronomically, at Saturday, 20 March 2021 at 09:37 UTC, the moment the Sun crosses the celestial equator heading north. That is day 79 of 2021. Meteorologists use a different convention entirely, fixing spring to 1 March through 31 May so that seasonal statistics line up with whole calendar months. Neither definition is wrong; they answer different questions. All four turning points of 2021 have passed.
When is the longest day of 2021?
The June solstice, Monday, 21 June 2021 at 03:31 UTC, when the Sun reaches its northernmost point. North of the equator that is the longest day and the shortest night of the year; south of it, the exact reverse. The December solstice on Tuesday, 21 December 2021 at 15:59 UTC mirrors it. Between the two, the Sun's noon height at any fixed latitude swings through the full range it will cover all year.
| Turning point | Date | Time (UTC) | Weekday | Day of year |
|---|---|---|---|---|
| March equinox | 20 March 2021 | 09:37 UTC | Saturday | 79 |
| June solstice | 21 June 2021 | 03:31 UTC | Monday | 172 |
| September equinox | 22 September 2021 | 19:20 UTC | Wednesday | 266 |
| December solstice | 21 December 2021 | 15:59 UTC | Tuesday | 356 |
How long is each season in 2021?
The four seasons are not equal. Measured between the turning points, the longest in 2021 is summer at 93.66 days and the shortest is winter at 88.98 days, a difference of 4.68 days. The cause is the Earth's elliptical orbit: it moves fastest when closest to the Sun in early January and slowest when furthest away in early July, so the arc between the September equinox and the March equinox is crossed more quickly than the other half.
| Northern season | Southern season | Begins | Time (UTC) | Weekday | Length in days |
|---|---|---|---|---|---|
| Spring | Autumn | 20 March 2021 | 09:37 UTC | Saturday | 92.75 |
| Summer | Winter | 21 June 2021 | 03:31 UTC | Monday | 93.66 |
| Autumn | Spring | 22 September 2021 | 19:20 UTC | Wednesday | 89.86 |
| Winter | Summer | 21 December 2021 | 15:59 UTC | Tuesday | 88.98 |
Why do the season dates move?
A tropical year, one March equinox to the next, is about 365.2422 days rather than a whole number. Each ordinary calendar year therefore ends roughly six hours before the Earth has finished its circuit, and the equinox drifts about six hours later on the clock. A leap day pulls it back by a whole day, which is why the date cycles across a three or four day window rather than creeping steadily. In 2021 the March equinox falls on the 20th and the December solstice on the 21st.
Clock changes in 2021
Daylight saving is statutory, not astronomical, and the rules differ. In the United States clocks go forward on 14 March 2021 and back on 7 November 2021, the second Sunday in March and the first Sunday in November. In the United Kingdom and across the European Union the change falls on 28 March 2021 and 31 October 2021, the last Sunday in March and the last Sunday in October. That leaves a gap of 14 days in spring when American and British clocks are an hour closer together than usual.
Questions
- When does spring start in 2021?
- Saturday, 20 March 2021 at 09:37 UTC, at the March equinox.
- When is the summer solstice in 2021?
- Monday, 21 June 2021 at 03:31 UTC.
- When is the winter solstice in 2021?
- Tuesday, 21 December 2021 at 15:59 UTC.
- Are the seasons the same length?
- No. In 2021 they run from 88.98 to 93.66 days because the Earth's orbit is elliptical.
Seasons in other years
More for 2021
Day of the year · Event countdowns · When is it? · Date calculator