Welcome Autumn! The September Equinox
Often during Astronomy related conversations I used to hear word equinox and then they used to say Fall equinox and the spring equinox. The word seems weird but it has the explanation in it. Equinox is derived from Latin word aequus (equal) and nox (night), because around the equinox the day and night are of almost equal lengths.
Equinox is on September 22 and March 20, so who cares
An equinox occurs twice a year (around 20 March and 22 September), when the plane of the Earth’s equator passes the center of the Sun. At this time the tilt of the Earth’s axis is inclined neither away from nor towards the Sun. So what is so important about Equinox.
In very simple terms, let me tell you what is the equinox from observation point of view
Equinox: The Sun from Solstice to Solstice Credit & Copyright: Tunç Tezel (TWAN)
It shows how the path of the Sun varies across the sky between Summer Solstice (top path), Fall Equinox (middle path) and Winter Solstice (bottom path).
As you can clearly see, the Sun is the highest in the sky during summer and lowest in the winter, lying in between at the fall equinox.
Once again, someone might ask you what so important about Equinox. What will we see in the sky? There is nothing to see in the sky in the equinox except the Sun and if you are near the equator, except to feel the heat, the blue sky and the dark nights 🙂 Usually during equinox it is hot during days and cool during nights at some places. What else can one benefit from Equinox?
Significance of Equinox
1. Sun will rise at exact East and will set at exact west if you are near the equator. You might be wondering that the Sun always sets at exact west and rises at exact East, this is not the cast. The below photo will explain it well.
The Sun rises due east and sets due west on the equinoxes in March and September. At other times of year it comes up and goes down somewhat to the north or south. This illustration is drawn for mid-Northern latitudes. Sky & Telescope illustration.
2. In northern hemisphere, the September equinox marks the start of Autumn and March equinox marks the start of Spring. In southern hemisphere it is opposite.
3. If you were standing on the equator, the Sun would pass exactly overhead at midday.
So a very simple concept, yet an important one and easy to understand.
Sun Set captured on the Equinox
Sun set on Fall Equinox September 22. Photo by Author
