• Stars & Science Austin

Spring Equinox

Written by: Lucia Brimer

In this month when all the news for the past couple of weeks has been about the pandemic coronavirus, I thought I would talk about something less disturbing, that happens every year at this time: the Vernal Equinox.

Have you ever wondered why we celebrate the first day of spring on or around March 21? And why does the date change sometimes to March 20 or 22? In many parts of the country, winter is still very much upon us through March and into April. And sometimes in Texas (like this year), spring seems well upon us by then.

Well, this date is known in astronomy as the Vernal Equinox. Vernal is from Latin vernus meaning “of the spring” and Equinox is from Latin aequus meaning “equal” and nox meaning “night”. So, on the Vernal Equinox, the day and night are approximately equal in time. So, why is that? What is going on here?

Well, it all has to do with the tilt of the Earth’s axis. As you have probably noticed, the Sun is up longer in the summer than in the winter, and the weather is much hotter in the summer too. It is hotter in the summer because the Sun is up longer and because it rises much higher in the summer. However, have you noticed that the Sun does not rise and set due east and due west all year long?

In the picture above (from Stellarium planetarium program, which is freeware open source software and can be downloaded from there are 2 lines tracing through the sky. The blue one is the Celestial Equator, which is just an extension of the Earth’s equator. It goes from due east to due west. The red one is the Ecliptic, which is the apparent path of the Sun during the year. The ecliptic goes through the constellations of the Zodiac (Pisces, Ares, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, and Aquarius). In the above diagram, the Sun is at the intersection of the 2 lines. This occurs on the Vernal Equinox (and also on the Autumnal Equinox, in September). The above image is at about noon, when the Sun is highest in the sky.

This image is at sunrise. You can see that the Sun rises due east on the Vernal Equinox, because the Sun is at the intersection of the Equator and the Equinox.

This image is at sunset. The Sun sets due west on the Vernal Equinox. As seen from space, the Sun shines directly on the Earth’s Equator, and all over the Earth, the day and night are approximately 12 hours long each.

If you can see the date above, it is on March 19, 2020. Yes, this year the first day of Spring is on March 19. It is the earliest spring equinox date since 1896 – 124 years ago! The Equinox date shifts a bit for several reasons:

1. The Earth’s orbit around the Sun is not a circle, it is an ellipse,

2. 365 days is not evenly divisible by 4 (seasons), and

3. The Earth’s tilted axis wobbles a bit, a movement known as precession.

So, get ready on March 19 for the earliest Vernal Equinox in over 100 years. From then on, the days will get longer and hotter, until we are back into another hot Texas summer. I will tell you about the Summer Solstice and the first day of Summer in June. Meanwhile, don’t try balancing an egg on its end to figure out the exact moment of the Equinox. That is just a superstition. It is not any easier to do on the equinox than on any other day of the year

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