darkoshi: (Default)
This morning, I started wondering why the eclipse will be seen on the west coast before it is seen on the east coast.

I know that the earth spins counter-clockwise (towards the east) when looking at it from above the north pole. And that the moon travels around the earth in the same counter-clockwise direction. And that the earth spins relatively faster. It does a complete rotation (360 degrees) in 24 hours, whereas the moon only travels 1/28th of the way around the earth (360 / 28 ~= 13 degrees) during that time.

So how can the moon's shadow travel from west to east? Isn't the earth spinning into the shadow and out of it in a clockwise west to east direction (the same as it always spins), and so the shadow should appear to move from east to west, just like the moon appears to do in the sky?

According to the answer on PhysLink.com, it has something to do with the moon's orbital velocity being greater than the earth's. But according to this orbital velocity formula, it seems that anything closer to the earth (ie. the earth's surface) would have a greater orbital velocity than something further away (ie. the moon). So that answer seems to be wrong or badly worded, maybe. Of course, if we simply consider velocity, the moon does travel a further distance through space than the earth's surface does, in the same amount of time. But what does that have to do with the eclipse? If it were a race, the earth would still win, rotating faster than the moon revolves.

Here's another page (cached, as the original eclipse2017.org page isn't responding - the website must be swamped) that tries to answer the question. Some of the commenters on that page seem to have the same confusion as I do.

Here's another page (Washington Post) that tries to explain it. Again talking about the speed of the moon compared to the earth.

Watching the various NASA visualizations didn't explain it well to me, because of how the videos keep shifting perspectives. Even in this animation, it looks like the sun must be moving from behind the viewer, to make the shadow move like that.

I think I may finally sort of understand it, but my explanation doesn't match any of the answers I read. So it's probably wrong. But... as the moon moves across the sun from right to left (as seen from the earth while facing south), it's shadow as seen from the earth changes direction. First it points towards the west, then straight, then towards the east.
And while the moon itself, from the earth's perspective, doesn't move far in the sky (and due to the earth's rotation, even appears to be going to the west*), it's shadow moves much faster... that must be why the answers keep mentioning the speed of the moon.. they must be trying to say that the speed of the moon's shadow across the face of the earth matches the moon's speed in space. I suppose that is logical, even though it isn't very intuitive to me**.

* But the sun appears to move to the west faster than the moon, so the moon does still cross the sun from right to left, even though they are both moving to the west.

So the moon's shadow moves quickly from the west to the east.

Right? Maybe? Sort of?

** Because the moon doesn't move in a straight line, but rather circles the earth. And those x-thousand miles per hour it moves up in space only correspond to y-hundred miles down on the earth... Oh jeez, now I'll start doubting my above explanation again...

Ok, thinking about it more. The shadow moves west to east like I explained above, because the moon crosses the sun from west to east. The speed of the moon through space around the earth affects the speed of the shadow, but it's not a direct x = y equation. The faster the moon moves across the face of the sun, the faster the shadow sweeps across the land from west to east. Since the shadow is sweeping through an arc (sort of), the far end of the shadow will pass a different distance during that time, depending from how far away you measure it... which for us is based on the distance between the moon and earth. So the speed of the shadow depends on that distance, and on the speed of the moon's revolution, and on the speed of the earth's rotation, and the size of the earth, etc. And it is complicated more because the moon moves in an elliptical orbit, not just straight past the sun, etc.

Now it makes sense to me. If I'm wrong, feel free to tell me which of my logic is wrong.


On a related topic, how long will totality last, across the U.S.? It will start on the west coast around 10:17am (1:17pm eastern time). It will end on the east coast around 2:48pm (eastern time). So for one and a half hours, the shadow will sweep across the country, from coast to coast.

Within that time period, based on the 3 to 4 hour time difference between the coasts, the earth only rotates about half the same distance.

But actually, the earth and the shadow are moving in the same direction... so if the earth weren't turning, the shadow would traverse the distance even faster.

2017/08/16 Corrected some words above. I was mixing up the words "right" and "left", even though I was visualizing it correctly. I'm used to thinking of the west coast as on the left side and the east coast on the right side. For the above, my perspective is from the center of the county looking south. So the west coast is on the right, not the left.
Although since the sun will be pretty high overhead during the eclipse, "left" and "right" aren't good words to use to begin with.
darkoshi: (Default)
My mom called to tell me that she heard that Jupiter is bright tonight - at opposition - and that it might even be possible to see its moons using binoculars. I thought, fat chance of that for me, but I went and looked. First I had to figure out how to adjust the focus on the binoculars; it's been a long time since I used them, and I was turning the diopter adjustment by mistake. Then, when I was looking at Jupiter, I did see another fainter object to the west-south-west (ie, left-bottom-left) of it, about 8 Jupiter-widths away. Could that be a moon? Based on this diagram, it's probably the star Theta Virginis.

Tonight, I kept having a problem with the binoculars, seeing double. I'm not sure if it is an issue with them or my eyes. To begin with, it wasn't double. Then, when it was, every once in a while, the double vision went away.

Since the moon is almost full, I looked at it through the binoculars too. Wow! Maybe I've never looked at it through binoculars before? That is the clearest I've ever seen the moon, that I can recall. The craters and lines/rays and texture/contours (towards the bottom) are visible, and even a crater sticking out on the left edge between light and dark.

Curiosity rover shows new signs of wheel wear - it's still sending back photos from Mars!? Wow, I didn't realize we still had contact with it.

By the way, it's weird that the moon always looks black & white / grayscale, when the rest of the universe is in color.

astronomy! hail!

Friday, May 23rd, 2014 09:49 pm
darkoshi: (Default)
Why do these articles seem so amazing* to me tonight?!

NASA discovers 715 new planets (including an earth-sized one in a habitable zone)

Astronomers find first asteroid with rings

Dwarf planet discovered at solar system's edge

And regarding tonight's meteor shower:
Meteor shower approaches in time for holiday weekend - not just any meteor shower, but a *new* one!

*I know! It's because I got to see quarter-sized hail thunking down out of the sky, just as I was ready to leave work! It was making loud ka-plunking noises on the overhead skylight/ceiling! And then when that ended, there was awesome mist swirling on the pond! And then the sun was so beautifully orange amidst the dark cloudy sky! And then after getting home, there was a little bit more hail, and one of those stationary cloud lightning light shows in the sky!
darkoshi: (Default)
If the Moon was only one Pixel - a tediously accurate scale model of the solar system. Wow. Just wow. I recall being similarly impressed by another scale model, which I thought I had also linked to, but I can't find that one any more...

Ah, here it is - an image showing the earth and moon, and distance between them, to scale.

(no subject)

Thursday, January 3rd, 2008 11:52 am
darkoshi: (Default)
Quadrantid meteor shower tonight - best viewable from western Europe and the Eastern U.S./Canada.

This page has a sky-chart of the radiant location.

It's going to be vewwy vewwy cold here tonight though, not sure if I will venture outdoors long to peek at the sky.

hair milestone

Monday, November 19th, 2007 12:18 am
darkoshi: (Default)
Today was the first day since I started growing my bangs out that my hair felt comfortable enough not to have to pin it back out of my face. In other words, it wasn't scratching my eyeballs or tickling my nose.

It sure is nice to be able to pull shirts over my head again, without it messing up the pins in my hair. And it looks much better without the pins.


I can see the sun as it rises from the yellow room window.

(The sun below the horizon is a window reflection.)
It is surprising how fast the sun rises - on the day I took the above photo, the sun was only halfway over the horizon when I first looked out the window, and that is what I wanted to capture. But in the minute or so that it took me to get my camera from the other room, it had already risen all the way over the horizon, as can be seen in the photo.

I will take a photo every week or so until the solstice, so I can see how the sun moves along the horizon, and what the southernmost sunrise point is... Maybe I'll try to figure out where it should rise on the solstice, based on this data, and then see if I'm right or not.

sunshine stuff

Sunday, May 21st, 2006 02:49 pm
darkoshi: (Default)
Starting in 2007, Daylight Savings Time in the U.S. will start earlier and end later. We will have about 3 more weeks of DST. Our start and end dates still won't match up with Europe's.

From Wiki... "Clocks will be set ahead one hour on the second Sunday of March instead of the current first Sunday of April. Clocks will be set back one hour on the first Sunday in November, rather than the last Sunday of October. ... This will affect accuracy of electronic clocks that had pre-programmed dates for adjusting to daylight saving time. The date for the end of daylight saving time has the effect of increasing evening light on Halloween (October 31)."


My compass doesn't seem to consistently point in the same direction for North. Sigh. How can I use my compass, when I can't even be sure it's pointing in the right direction?

Columbia, SC data
(latitude 34 degrees north, longitude ~81 degrees west)

- Magnetic North is 6 degrees 40' west of geographic North

- On the summer solstice, sunrise azimuth is at ~ 61 degrees, and sunset is at ~298 degrees.
- On the winter solstice, sunrise azimuth is at ~118 degrees, and sunset is at ~241 degrees.

Ie., the sunrise angle varies between 0 and ~29 degrees north and south of Due East
and the sunset angle varies between 0 and ~29 degrees north and south of Due West.

- The sun's highest altitude in the sky is...
on the summer solstice at 80 degrees,
on the equinox at 56 degrees,
and on the winter solstice at 32 degrees.

- In the summer, earlist sunrise is at 6:12am and latest sunset is at 8:40pm (Daylight Savings Time).
- In the winter, latest sunrise is at 7:30 am and earliest sunset is at 5:15pm (Standard Time).