Equinox sunsets mark due west in the sky
12 comments Print Me Email to FriendTonight's Sky for Thursday, Mar 20 2008
The March equinox – when the sun crosses celestial equator, moving from south to north – happened during the night last night according to U.S. clocks. It happened at 5:49 Universal Time, or 12:49 a.m. Central Daylight Time this morning … or late on the evening of March 19 for time zones further west in the U.S.
The equinox is a hallmark in Earth’s orbit, but it’s also an event that happens on the imaginary dome of Earth’s sky. The celestial equator is an imaginary line on our sky. It wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator. All these imaginary components . . . and yet what happens at every equinox is very real, as real as the sun’s passage across the sky each day and as real as the change of the seasons.
Our ancestors couldn’t have understood the equinoxes as we do. They didn’t understand them as events that occur in the course of Earth’s yearly orbit around the sun. But if they were observant . . . and some were very observant indeed . . . they surely marked today as being midway between the sun’s lowest path across the sky in winter and highest path across the sky in summer.
If they thought in terms of four directions, they might also have learned a fact of nature, that occurs whenever there’s an equinox . . . whenever the sun crosses the celestial equator. Since the celestial equator intersects the horizon at due east and due west, the sun rises due east and sets due west on the day of the equinox, as seen from everywhere on the globe.
Larry, that was fascinating. Even having read “Galileo’s Daughter” (a book I highly reccomend), I don’t remember reading about how he reasoned that Venus orbited the sun. Was he able also to calculate from this the distances of earth and venus from the sun? Seems like a little trigonometry just might do it. At least their relative distances.
It’s good for all of us to remember, too, that this brilliant man believed that the tides were caused by the ocean “sloshing” as the earth turned. I don’t think he ever figured out that the tides were associated with the moon; possibly because Isaac Newton wouldn’t explain gravity for another century or so. Also, since the high tide trails the moon’s position by something like ninety degrees (I’m not sure of that number), the linkage isn’t all that obvious. Anyway, when I start believing I know everything, I remember Galileo and his tides theory.
Ray
Ray, Galileo really wasn’t all that much a mathematician, certainly not like his contemporary and correspondent Kepler, nor Newton later. I suspect that he used geometrical reasoning (charts) in drawing his conclusions. Basically, in the old Ptolemaic (geocentric) view of the Solar System, everything had to orbit Earth. Galileo however, believed in the Copernican System in which all the major planets orbited the Sun, which of course is what we now know is true. Galileo reasoned that if Venus orbited Earth, the phases seen through his telescope would be just like the Moon’s phases. For example, when the Moon is full, it is opposite the Sun. However, in a telescope when Venus appears most nearly full, it is closely in line with the Sun and never is seen opposite our star. [Note that this simplifies the Ptolemaic System a bit, but gets the main point across.] So Galileo took this as evidence for the heliocentric viewpoint in which the other planets (and Earth) orbit the Sun, rather than everything going around our world.
LS
BTW, I forgot to mention, but Galileo did not know the distances to the Sun or planets. He had some idea of the relative distances (which Copernicus had figured and Kepler later elegantly linked with their orbital periods around the Sun), but he did not know the true distances. For example, he knew or at least had reason to believe that Mars was about 1.5 times as far from the Sun as the Earth, and that Jupiter was about 5 times, but since he did not know the exact distance between the Earth and the Sun, he didn’t know any of the true distances.
LS
Thanks for the information, Larry.
It’s humbling, how much these early scientists managed to deduce with just a few primitive instruments.
Dr. Barbara Becker admirably explains how Copernicus computed the Relative Distances of Mars, Venus and Mercury from Earth on her online “Exploring the Cosmos” history course!
Hi Team !
I have a question please. Why do we suddenly have an extra 20 minutes daylight hours ( In my case at sunny Kinloss, Scotland)at the equinox, followed by 8 minutes less daylight the next day? I also notice that leading up to the equinox, daylight hours have only increased by up to 6 minutes per day. Thank you. Spencer Julian. (ps great web site!)
Hi Spencer,
I checked the sunrise/sunset times for your latitude (Kinloss, Scotland: 57 degrees 39’ N latitude, 3 degrees 34’ W longitude) via the US Naval Observatory. It appears to me that on the day of an equinox, you have roughly 12 hours and 15 minutes of daylight. The daylight increases by about 5 minutes per day leading up to and following the March spring equinox, and decreases by about 5 minutes per day (at your latitude) around the September autumnal equinox. If I haven’t answered your question to your satisfaction, please feel free to contact me again!
Bruce
Hello Bruce.
Thank you for your prompt reply. From day to day I check the weather forecast at various places around the planet, on www.bbc.co.uk/weather/5day. The web site gives me other useful information as well – such as sunrise and sunset times. The sunrise/sunset times I have noticed, doesn’t make sense to me during the period around the equinox. There seems to be a big jump of approximately 27 minutes extra daylight on Friday, followed by 8 minutes less daylight the following day (for Kinloss). I have checked with Berlin, Paris, London etc and they seem to have similiar comparisons.The New York and Washington times appear to be a bit odd as well. Could the BBC be mistaken? Thank you. Spencer
Spencer,
I checked out the BBC page, and yes, I believe the BBC is mistaken on the sunrise/sunset times. Good eye! I agree with you that the sunrise/sunset times don’t make sense!
Bruce
Hey Hey Hey my name is J from a gobbledee ra to the sob of the day?
Just a question, Are you still able to see Saturn tonight? I wasn’t able to see it the past couple days because of overcast weather.. I’m on EST right now, and basically wondering if I can see Saturn tonight by the Moon
Bob,
Yes, you can still see Saturn. At nightfall tonight (Thursday, March 20), you’ll see it as a bright, steady point of light roughly 20 degrees above the moon. For reference, your fist at an arm length from your eye spans approximately 10 degrees of sky.
Do you know how to find Polaris, the North Star, with the pointer stars of the Big Dipper (Dubhe and Merak)? If so, draw a line through the Big Dipper pointer stars in the opposite direction to find Saturn. Saturn will remain in the evening sky till August 2008.
Bruce