Earth shadow

[multiweb (Marc)]

Quick question for the celestial mechanics out there. What is the motion of the earth shadow in reference to the star field in the lunar eclipse area. I had a look in sky safari and it moves a fair bit over the 4 or 5 hours or so. Can it be considered linear or is it something else?

[RB (Andrew)]

Originally Posted by multiweb

Can it be considered linear or is it something else?

Interstellar.

[Sunfish (Ray)]

Interesting question. Isn’t the earths shadow path the ecliptic.?

Maybe not linear as the shadow and starfield rotation is the earths orbit and the sidereal day the earths rotation? Too complicated for me.

https://skyandtelescope.org/observin...ipses-in-2021/

Solar eclipse in December sounds interesting.

Edit. You can do an “airborne intercept of the totally eclipsed sun” sounds like Douglas Adams

[multiweb (Marc)]

Originally Posted by Sunfish

Isn’t the earths shadow path the ecliptic.?

You're right. Simplest way to think about it.

[Sunfish (Ray)]

Except for the orbit wobbles and the seasonal field tilt , the daily non linear relationship must the same as the difference between the solar day and a sidereal day, only a few minutes in time. Maybe you would not notice that but difference in curved path looks weird.

I turned on the ecliptic lines in Stellarium along with the equatorial . It is interesting to watch the centre of the ecliptic rotate around the CP as the time hour is advanced like two cages almost locked together.

Originally Posted by multiweb

You're right. Simplest way to think about it.

[multiweb (Marc)]

Originally Posted by Sunfish

Maybe you would not notice that but difference in curved path looks weird.

It definitely curves over. I'm asking because I'm trying to put a widefield annotated animation together and ran into problems matching the shadows with the starfield I shot the day after. I managed to register all the panels during totality because I had enough stars to align and that's when I noticed discrepancies between the photos and the static umbra and penumbra I had assumed in the series.

[N1 (Mirko)]

I would add that because the earth umbra is relatively short, there's going to be a deviation of its apparent location from the antisolar point that is a function of the observer's distance from the sun-earth axis. You'd basically see that as the moon's parralax relative to the star background.

[multiweb (Marc)]

Originally Posted by N1

I would add that because the earth umbra is relatively short, there's going to be a deviation of its apparent location from the antisolar point that is a function of the observer's distance from the sun-earth axis. You'd basically see that as the moon's parralax relative to the star background.

Good point. I hadn't thought about that either. It's a relatively short cone.

[N1 (Mirko)]

I should qualify that, moon parallax only when considering the distance at which the moon intercepts the umbra. The shadow's apex is further out of course. It still deviates however as it's not indefinitely long.

[Sunfish (Ray)]

I tried to recover my little moon eclipse sequence from before the tripod blew over but I gave up.

The eclipse curves over because the umbra is not falling on a static plane but on a surface which is rotating into view in a different plane to the earth rotation and orbit combined. The moon appears to us to be going straight over but in relation to the sun it is receding into the pointy end of the umbra.

The time period of the movement of the umbra on the moon , not the umbra itself against the stars, is varying constantly . Check an eclipse animation zoomed on to the moon like the one on Stellarium hour by hour with ecliptic lines and equatorial lines and you can see the path.