Today at 09:37 UTC is the vernal equinox, the official start of spring in the Northern hemisphere where I am located. On this day the length of daylight and darkness at night is nearly the same and because it is not exact articles will be written and people will argue about it. Lost in all this is the fact that the equinoxes do not happen at exactly the same time every year. Look this up, last year the spring equinox was at 03:50 UTC on March 20th, almost six hours earlier than this year. Next year the spring equinox is at 15:33 UTC on March 20th, almost six hours later than this year. This highlights a problem with our calendar system: its messy to use for noting a position in space, where Earth is in its orbit around the Sun.
Simple geometry can give us an answer. Since the Earth’s orbit is nearly circular we can use the fact that there are 360 degrees in a circle to note where Earth is. This position is called solar longitude (Ls), denoted λ⊙, and is expressed in degrees. Knowing where Earth is in its orbit around the Sun is particularly useful in meteor science. At certain points in our orbit Earth crosses a stream of particles that produce what we call a meteor shower. So how do we create a reference for that not based on a calendar? We start by using the equinox.
The equinox is defined as the moment the plane of Earth’s equator passes through the geometric center of the Sun’s disk. This moment of equal day and night, or a precise point where the sun rises or sets on the horizon has been used for a long time in human history. As the Earth orbits the Sun it crosses this point because Earth’s rotation is tilted. In the spring equinox the Sun moves above the Earth’s equatorial plane and in fall it moves below that plane. These moments can be used as a precise reference if another point is used, a star(1).
Today at 16:32 UTC solar longitude is zero degrees.
Choosing the spring equinox as the start, or zero degrees, is an arbitrary choice. I do not know why the choice was made, historical significance? Perhaps a reference star is better in spring then fall? Research for another day. But the point chosen is an exact spot, a reference we can use to start marking the circle that represents our orbit.
When a meteor shower happens is noted by solar longitude to mark the position when Earth moves through the center of the orbit of particulate debris causing the shower. In other words, typically when the maximum of a meteor shower occurs. For example, the peak of the Perseid meteor shower happens at 140 degrees λ⊙. This translates to August 12th at 19:17 UTC for this year, in 2022 it will be August 13th at 01:32 UTC.
The reason why the equinox does not happen on the same date time every year is our Julian calendar system has 365 days in a year (366 during a leap year). In a year the Earth completes one revolution around the sun, or 360 degrees in a circle. This is the problem: 365 days in a year verses 360 degrees in a circle, our way of tracking dates and time gets out of sync with the exact location we are in space.
- We have to go a step further to be precise in choosing this point because everything in the universe is in motion. So we use a specific epoch ( https://en.wikipedia.org/wiki/Epoch_(astronomy) )