by Anthony Barreiro » Thu Sep 20, 2012 9:01 pm
Remo wrote:This picture has me calling into question one of my long ago learned "facts" -- that the sun always rises directly in the east at the equinoxes. Clearly, since the scope of the picture covers less than the time period between the vernal and autumnal equinoxes, the sun will not line up in the same place at the equinoxes, i.e., directly towards the east.
And yes, I've known for a long time about the Analemma and the reasons for it (tilt of the earth and eccentricity of earth's orbit). I just didn't put 2 and 2 together. I'll just have to dust off Kepler's laws and work out the 3d implications to work out the amount of the anomaly. Shouldn't be to hard.
Hi Remo. The Sun does rise directly east at the equinoxes,
if your horizon is perfectly flat. For those of us in the northern hemisphere, if there are hills, trees, buildings, etc. on your horizon, the Sun will be slightly south of due east by the time it rises above your visible horizon. With hills, etc., you will see the Sun farther south at the Spring equinox sunrise than the Fall equinox sunrise, because the ecliptic angles slowly away from the horizon at the Spring equinox sunrise, and more steeply away from the horizon at the Fall equinox sunrise. (Everything is reversed in the southern hemisphere.)
Also, the first of these exposures was taken on April 2, two weeks after the Spring equinox, and the last exposure on September 16, a week before the Fall equinox, so the rising Sun was already south of due east in the first exposure, and was still north of due east in the last exposure. The rising Sun changes position along the horizon most quickly during the weeks before and after the equinoxes, so you're going to see slight movement from one day to the next.
I've been noting the movement of the rising Sun along my hilly urban horizon in honor of the approaching Fall equinox. When it gets to the flagpole on the schoolground, I'll know it's time to celebrate!
By the way, this is an awesome picture! Venus transiting the Sun is a wonderful Easter egg!
[quote="Remo"]This picture has me calling into question one of my long ago learned "facts" -- that the sun always rises directly in the east at the equinoxes. Clearly, since the scope of the picture covers less than the time period between the vernal and autumnal equinoxes, the sun will not line up in the same place at the equinoxes, i.e., directly towards the east.
And yes, I've known for a long time about the Analemma and the reasons for it (tilt of the earth and eccentricity of earth's orbit). I just didn't put 2 and 2 together. I'll just have to dust off Kepler's laws and work out the 3d implications to work out the amount of the anomaly. Shouldn't be to hard.[/quote]
Hi Remo. The Sun does rise directly east at the equinoxes, [i][b]if[/b][/i] your horizon is perfectly flat. For those of us in the northern hemisphere, if there are hills, trees, buildings, etc. on your horizon, the Sun will be slightly south of due east by the time it rises above your visible horizon. With hills, etc., you will see the Sun farther south at the Spring equinox sunrise than the Fall equinox sunrise, because the ecliptic angles slowly away from the horizon at the Spring equinox sunrise, and more steeply away from the horizon at the Fall equinox sunrise. (Everything is reversed in the southern hemisphere.)
Also, the first of these exposures was taken on April 2, two weeks after the Spring equinox, and the last exposure on September 16, a week before the Fall equinox, so the rising Sun was already south of due east in the first exposure, and was still north of due east in the last exposure. The rising Sun changes position along the horizon most quickly during the weeks before and after the equinoxes, so you're going to see slight movement from one day to the next.
I've been noting the movement of the rising Sun along my hilly urban horizon in honor of the approaching Fall equinox. When it gets to the flagpole on the schoolground, I'll know it's time to celebrate!
By the way, this is an awesome picture! Venus transiting the Sun is a wonderful Easter egg!