Sun and Earth Equinoxes and solstices
Equinoxes and solstices
Seasons come from tilt, not from distance
Four turning points set by Earth’s tilt, governing day length and the seasons.
- Axial tilt
- About 23.4°
- Equinoxes
- Sun overhead at the equator
- Solstices
- Sun overhead at a tropic
01 / Overview
Meet Equinoxes and solstices
Earth’s rotation axis is tilted about 23.4° to its orbital plane, and keeps pointing the same way all the way round the Sun. The subsolar point therefore travels back and forth between the tropics: the two crossings of the equator are the equinoxes, and the northern and southern extremes are the solstices.
Are day and night really equal at an equinox?
Close, but not exact. Refraction lifts the Sun into view while it is still geometrically below the horizon, and sunrise and sunset are timed from the upper limb, so daylight runs a few minutes longer than night. The date when they truly match is the equilux, usually a few days to either side of the equinox.
The solstice and the earliest sunset
The winter solstice is the shortest day, but not the day of the earliest sunset. The gap between apparent and mean solar time — the equation of time — puts the earliest sunset before the solstice and the latest sunrise after it. It is also one reason a sundial and a clock disagree.
02 / Key data
Core parameters
- Axial tilt
- About 23.4°
- Equinoxes
- Sun overhead at the equator
- Solstices
- Sun overhead at a tropic
- Exactly equal day and night
- The equilux, not the equinox itself
- Recurrence
- Four times a year
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- Near an equinox the Sun rises almost due east and sets almost due west — useful for checking directions.
- Record the Sun’s position at the same clock time each day and a year draws an analemma.
- Around a solstice the Sun’s altitude changes most slowly — the word itself means the Sun standing still.
04 / Sources