One of the ancient questions about the solar system is “how do we know that it’s in motion”? Further to that perhaps would be “what part of it is moving and what part isn’t”?
Today, we have a very good understanding of the celestial waltz of the planets. However, despite the fact that the astronomers have a lot of it figured out, there is still a large gap in understanding for the average sky watcher. So allow me to explain something that I’ve been noticing about the night sky recently, that being the fact that the days are getting shorter already!
Have you noticed how it’s getting dark earlier now? With the advancing seasons from summer into fall, this becomes one of the most obvious markers. But why exactly does it happen?
To astronomers, the beginning of the celestial year is the first day of spring, also know as the vernal equinox (equal night/equal day). Usually this is on or about March 20th and would be the inspiration for that 70s era song, The Age of Aquarius. On that particular day, the Sun’s rays strike Earth directly, or perpendicularly, at the equator. This would be inconsequential, except that the Earth is tilted by 23.5 degrees to vertical. So as the Earth orbits the Sun, this changes the angle of attack by sunlight. This alignment allows for there to be seasons on Earth.
Each day as the Earth continues on it’s march around the Sun, the sunlight strikes Earth perpendicularly further and further north. This continues until the first day of summer, when the advancing sunlight “stops” moving north, which is what we call the Summer Solstice (translating into sun stops). This changing of the angle of “attack” causes the amount of daylight to increase by about 4 minutes per day (2 minutes in the morning and 2 in the evening).
So now if you’ve picked up on the pattern, from the first day of summer and onwards, the length of daylight begins to decrease. At first, it isn’t very obvious, but now as we get closer to fall, suddenly it may have become quite noticeable. For some, this brings excitement at the thought of crisp fall days and pumpkin spice lattes. For others, it means the start of the school year and the end of a long vacation. Either way, the Earth holds back it’s motions for nobody and the evidence is most obvious in the darker skies.
In order to actually picture what’s happening, imagine if you could get into a space helicopter and be lifted way up “above” the solar system. Of course, in space the rotors would generate no lift at all, being that space is without air pressure, but continue to imagine. When you got high enough, you would see the Sun in the centre with many orbiting bodies whirring around it. They would all be going in the same direction (counter-clockwise), with the inner objects moving faster than the outer ones.
Picture there being a start line at a certain point, sort of like on the track at the Olympics. That would be where Earth was on the first day of spring. Now almost 6 months later, the Earth is almost half way around the “track”. At the quarter way point, the Earth’s tilt would have been pointed directly at the Sun, but now the Sun’s rays are more and more focussing on the Earth closer to the equator.
It is indeed hard to picture. Our perspective from Earth is quite limiting, but there have been times when the greatest discoveries in science came from imagining the very small or the very large. In fact, Albert Einstein himself made most of his theories through these sort of mind experiments, simply because there was no other way to test things like relativity at that time.
However, whether you can picture the alignment of the planet relative to the Sun or not, I encourage you to continue to look up each night. Notice the stars and how they seem to be in a new spot each night. Some will move out of sight over time, while others will come into view. The summer constellations giving way to the winter. And if you have any questions about this, leave me a comment. I’ll try to answer your question or perhaps use it for a future blog post.