Time

Equinox & Solstice

4.12 - Understand the astronomical significance of equinoxes and solstices

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4.13 - Understand the variation in the Sun’s apparent motion during the year, particularly at the equinoxes and solstices

Above the Equator is the Northern Hemisphere; below it is the Southern Hemisphere. Imagine if the Equator is projected into space. This would be called the celestial equator.

The path the Sun appears to take over the course of a year is called the ecliptic. This is the path of the Earth's orbit.

The Earth's axis is titled at an angle of 23.5 (22.26)° to the ecliptic.

Every day the Sun appears to move 1 degree eastwards. This is because there are 360° in a circle and 365 days in a year.

Between December and June, the Sun is in position above the celestial equator and the Northern Hemisphere receives more daylight. Between June and December, the Sun is below the celestial equator and the Southern Hemisphere receives less daylight.

During the summer months in the Northern Hemisphere the North Pole receives sunlight for 6 months and the Sun does not set. During the winter there is darkness for 6 months. The opposite is happening at the South Pole in this time.

Let's look at some 'lines' to understand this. The equator is an imaginary line around the widest point of the Earth.

There are two other lines of latitude which we need to learn. These are the Tropics of Capricorn and Cancer. They are named after these constellations as they are named after the constellation the Sun is in at different parts of the day.

Around 21st March and 21st September the Sun passes over the celestial equator. If you were standing on the Equator the Sun would be directly above your head at midday. These dates are called the equinox, when day and night are of equal length around at every point on Earth.

Around 21st December the Sun is at its furthest point southwards on the Ecliptic. If you were standing on the Tropic of Capricorn, you would see the Sun above you at midday.

Around 21st June the Sun is at its furthest point northwards on the Ecliptic. If you were standing on the Tropic of Cancer you would see the Sun above you at midday.

These dates are called the Solstice; when day is at its shortest or longest.

The equinox does not stay at the same time every year. Earth's axis wobbles slightly like a spinning top. This means that the Vernal Equinox is getting slightly earlier each year. This is called "Precession of the Equinoxes". The point at which the Sun crosses the ecliptic at the Vernal Equinox is called the "First point of Aries" and since our ancients observed it in Aries, it has shifted into the constellation of Pisces and in the not distant future will occur in Aquarius.