Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Sylvia Hewitt on 14/05/2010 01:30:02

Title: How does the path of the sun change?
Post by: Sylvia Hewitt on 14/05/2010 01:30:02: Sylvia Hewitt asked the Naked Scientists:

Hi Chris,

I know the Sun and planets are more or less in a plane, so follow an arc through our sky, with the Moon following an arc that's just a bit off from that one (which I guess is why we don't always have an eclipse with every full Moon).

But since we are tipped, that arc must move in the sky during the day (which way is hard for me to figure out). And it also moves seasonally.Â

So the motion of the Sun must be along a path that is itself moving in the sky.Â Can you describe this motion? The Equinox must be that moment when the arc crosses due East at the horizon at the same moment the Sun is in that position.Â

But how did early civilizations figure out when that was, given that nobody's horizon is ever really at 0 degrees?

Thanks if you've read this far!

Cheers and regards

Sylvia

What do you think?
Title: How does the path of the sun change?
Post by: Geezer on 14/05/2010 05:29:26: Hi Sylvia,

It's probably best to describe this with a diagram, but I'll give it a shot.

The Earth orbits the Sun, so imagine the path the the earth moves along as being the edge of a disk. The surface of this disk is the plane of the Earths orbit.

The Earth rotates on its axis approximately once every day, but the Earth's axis is not at 90 degrees to the plane of its orbit. It's tilted over, but the amount of tilt and direction of the tilt remains fairly constant.

So, as the Earth goes around the sun, at a certain point, the North Pole is tilted away from the sun as far as possible. The happens at the Winter Solstice in the Northern Hemisphere. At the same time, the South Pole is tilted towards the Sun as much as possible, so it's the Summer Solstice in the Southern hemisphere.

At the two equinox points, the Earth's axis is neither tilted towards or away from the Sun. At those times the Poles are tilted towards the path that the Earth follows.
Title: How does the path of the sun change?
Post by: Sylvia Hewitt on 15/05/2010 21:06:55: OK, thanks, but it seems to me this reply just re-states my question.
Title: How does the path of the sun change?
Post by: Geezer on 15/05/2010 21:55:57: OK - I must not have understood your question. BTW, the Sun does not "move" in the sky - not relative to the Earth anyway.