The moment of an equinox is an exact alignment where the sun hits the spot where the ecliptic meets the celestial equator.
Let’s decode and unpack that together….staring with some basic astronomical definitions to lay our foundation…
Celestial Sphere – The sphere of the sky dome is called the celestial sphere. Each point of light in the night sky looks as if it is moving along a dome above us. Imagine you and I were sitting in a planetarium together, looking up at the domed ceiling. The objects in the universe are represented by points of light projected onto the ceiling. The second half of the sphere is the daytime sky “below” us, on the other side of the planet.
Celestial Equator – the celestial equator is a projection of the earth’s equator up onto the celestial sphere.
Celestial Body – a star/sun or planet, moon, asteroid or comet – and body that lives in space. They show up as dots of light in our night sky.
The Ecliptic is the pathway the sun makes across the sky. You can read more about it here:
Planet Pathways - Because of the way the solar system was formed – and the nature of creative and cosmic forces – most of our solar system orbits the sun on a plane. So the pathway of the planets and the moon closely follow this same ecliptic path through the sky.
It is worth taking note that the moon deviates from this path regularly. With its monthly fluctuations, these lunar standstills are a subject to explore later together….
Seasons and Tilt – Planet earth has a tilt on axis, which initiates and governs the cycle of our seasons. If the axis of rotation and the axis of revolution were set at the same angle, every day would be the same as far as length of day and night, and we would have uniform light levels and therefore seasons. It is the tip of the northern or southern hemisphere towards or away from the sun that makes winter and summer, spring and fall, cooler and hotter times of the year.
Because our planet earth has this tip to its rotational axis, the celestial equator flows at a different angle across the sky from the ecliptic. Here is a graphic to help you picture it:
The Equinox Moment –The Equinox is astronomically defined as when the Sun meets the point on the sky dome where the ecliptic crosses the celestial equator. This defines an astronomical new year.
These two images highlight this equinox moment:
Images from: https://earthsky.org/astronomy-essentials/why-equinox-sun-rises-east-sets-west-entire-globe/