This year’s astronomical beginning of spring, the first of two moments during the year when there are precisely 12 hours of daylight and darkness, occurs today at 04:30 Coordinated Universal Time (UTC). For those who reside in the US Northeast, that would be 00:30 (12:30 AM) EDT. Although there is nothing to “see”, per se, this year or any year, it is noteworthy in that it occurs during the nighttime hours since it involves the position of the sun on the ecliptic relative to the Celestial Equator, the imaginary path described by the projection of the earth’s equator onto the sky.
Why, you may ask, do the seasons change and what does it mean to say “the astronomical beginning of spring“, summer or any of the other seasons? Before we answer that question though, we must first consider that the earth is tilted by approximately 23.5 degrees from the vertical (or 66.5 degrees from the plane of the ecliptic), that it orbits the sun (as all objects do) in an ellipse and that its axis always points to the same points on the sky. Those points are the NCP (the North Celestial Pole) and the SCP (the South Celestial Pole), the imaginary points on the sky that are the extensions of the earth’s axis. It should also be noted that, paradoxically, the warmest season in the northern hemisphere, the summer, occurs at the earth’s aphelion, the point in the earth’s orbit that is farthest from the sun. So, contrary to what some may think, if its not the distance from the sun that is driving the seasonal changes, what is it?
Since the earth is tilted and its axis always points to the Celestial Poles, the altitude of the sun varies from day to day and from season to season. In the warmer months, the sun is at its highest in the sky; during the coolest months, the sun is at its lowest point in the sky. A simple thought experiment helps to understand this; imagine shining a flashlight on a flat surface, first directly over head and then at a very low angle. Note that the beam from the overhead position is round and concentrated on the surface and the beam from the low angle is elongated and spread out on the surface. This is what happens on a grand scale as the sun’s altitude changes from season to season as it orbits the sun. In general then, it is the response of the earth and it’s atmosphere to the changing energy received from the sun.
To answer our original question, we must also consider that the planets orbit the sun in an imaginary plane we call the Ecliptic. The one-dimensional analog to this two-dimensional orbital plane is the imaginary path the sun makes as it moves eastward by one degree each day of the year (360 degrees/365 days per year). We refer to that path of the sun as the “Path of the Ecliptic” (see the opening illustration at the top of this article, produced by the popular open source application, Celestia).
So, the beginning of each of the four seasons is as follows:
- Vernal Equinox – the sun transits the Celestial Equator ascending along the Ecliptic, marking the beginning of Spring; 12 hours of daylight and darkness and generally occurs on March 20 – 21.
- Summer Solstice – the sun reaches its highest point on the sky; longest day, shortest night and generally occurs on June 20 – 21.
- Autumnal Equinox – the sun transits the Celestial Equator, descending along the Ecliptic; 12 hours of daylight and darkness and generally occurs on September 20 – 21.
- Winter Solstice – the sun reaches its lowest point on the sky; shortest day, longest night and generally occurs on December 20 – 21.
The actual date varies from year to year but it generally occurs according to the schedule above. The schedule for this year:
- Vernal Equinox, today, 20 March, 04:00 UTC
- Summer Solstice, 20 June, 22:34 UTC
- Autumnal Equinox, 22 September, 14:21 UTC
- Winter Solstice, 21 December, 10:34 UTC
Imagination is more important than knowledge
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