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2017 Eclipse Blog / FAQ

Why does the eclipse go from West to East?

The Sun and Moon both appear to “rise” in the east, and “set” in the west.  But as you can see in our video of the Moon’s shadow sweeping across the USA, you see that the shadow plainly travels from west to east across the country!

This is a very good question, because it shows how Solar eclipses take us out of our normal experience. We are all used to seeing the Sun and Moon rise in the east and set in the west. But if you were situated HIGH above the north pole, the Moon would appear to move counterclockwise (i.e., from your right to left) in its orbit around the Earth. And it is THAT motion of the Moon moving in its orbit, dragging its shadow along behind it, that causes the eclipse path on Earth to move from west to east.

As you watch the total eclipse (you ARE in the path, right?!) you will see the “bite” that the Moon takes out of the Sun start on the “right” side of the Sun’s disk, then slowly grow as the Moon apparently moves across the face of the Sun – from right to left. Since you are looking up at the Sun (with your eclipse glasses on) while facing South, then West is to your right. So as the Moon itself moves from right to left, its shadow also goes from west to east.

NASA has just come out with an eclipse animation that shows this PERFECTLY!

You’re watching the Moon move IN ITS ORBIT, and not as a result of the Earth’s rotation.  And that’s pretty cool!

21 Responses to “Why does the eclipse go from West to East?”

  1. BillyBowlegs says:

    Thanks! I believe if the moon’s orbit were somehow suddenly arrested mid-eclipse, the direction of the umbra would reverse and go east to west as a result of the earth’s rotation. So the earth’s rotation has the effect of prolonging the time of totality. I wonder how long totality would be reduced if the earth did not rotate?

    • Admin says:

      Hi Billy – The math to give an exact answer is tricky, but thumbnailing it is pretty straightforward.

      If you mean the total amount of totality that the Earth receives as a whole, then the amount of totality (in the case of the 2017 eclipse, about 100 minutes) would not change. The Moon’s shadow would still take that long to move across the face of the Earth, because its motion is due to the Moon’s revolution around the Earth, irrespective of the fact that the Earth happens to be rotating underneath it.

      But please note that that path on the Earth’s surface would change! The fact that the Earth rotates is very important for the calculation of where on the Earth’s surface the shadow hits!

      But if you mean the amount of totality for a given spot on Earth, then let’s say a particular spot that is in the path for both rotating and non-rotating Earths gets 2 minutes of totality. At latitude 40N, the Earth moves in its rotation (“angular velocity”) let’s say 750mph. In 2 minutes, that’s 25 miles. The Earth therefore loses 25 miles worth of totality if it’s not rotating, which if the shadow is moving 2000mph is about 45 seconds – a decent amount! But you can see how complicated the actual math would be, because it depends on literally everything – the speed of the shadow, the latitude on the Earth, the angle of the Moon’s shadow, the duration of totality (eclipse magnitude) – lots of things.

      I would therefore say that a stationary earth would offer observers a “somewhat statistically significant” amount of totality loss – which would, of course, NOT offset the “extremely statistically significant” non-eclipse-related problems they would be experiencing at that point!

    • Admin says:

      Hi Billy – That is correct, but if the Moon were suddenly stopped, we would have bigger problems! Anyway, the effect of the Earth’s rotation is to increase the duration of totality about 20-40% (depending on a lot of things) in any given spot.


  2. Dadi Ratnagar says:

    We’re going to be present in the totality region in Oregon and would like to take pictures of the August Eclipse event. Please advice on what kind of camera to use with what kind of filters, what exposures, / speed, etc.. This is for still photography. If this is not the right place for these questions please forward or advice us on the correct email to write to.

  3. Why is there not as eclipse more often somewhere in the world? Realistically with the moon taking 28 days(672 Hrs) to orbit the earth as it makes its full rotation every 24 hrs. Wouldn’t the moon cast a shadow somewhere 14 of the 28 days?
    And if the moon traveled at exactly the same speed(fixed) to the earth the shadow would be west to east. Ad the spin of the Earth and it would easily be viewed going East to West No matter what the position of orbit in relation to the sun!
    In my opinion the book of Enoch explains it best… The sun and moon are the same size! The earth is flat and not spinning at over ther speed of sound and traveling through space 1.6 million miles a day! Really?

  4. I don’t know how but mayan and chinese astronomer were predicting solar eclipses accurately thousands of years ago,when the earth was flat. now that its round i’m sure Nasa uses the same technology?

    • Admin says:

      Hello –

      Some things that are known now were surely known then, and I cannot speak for any ideas the Mayans and Chinese had of the Earth being flat. Nowadays, we used a method of eclipse predictions developed by Bessel in the early 1800s, and refined by mathematicians through the years since. It involves spherical trigonometry and an iterative method, neither of which were known prior to Newton. Therefore, our predictions today are MUCH more accurate than those of antiquity, and they are proven correct each and every eclipse!

      Get to the path, and enjoy totality!


  5. Stuck on Reality says:

    Having grown up with a geocentric perspective it took me a while to understand why the path moves from west to east. I understood that the west to east orbital motion of the moon is what brings it into proper alignment to begin the eclipse, but that doesn’t address the effect of the earth’s rotation under the shadow. Knowing that the Earth is rotating at 15º per hour while the Moon’s angular velocity is only .5º per hour it seems intuitive that Earth’s rotation would be the determining factor.

    Then I started thinking from a heliocentric perspective, and imagined the moon transiting the face of the Earth as seen from the sun. Of course that makes it even easier to visualize how the motion of the moon rather than the Earth causes the alignment that results in the shadow (that’s always somewhere, of course) appearing on the Earth. What it also did was get me thinking about the moon’s orbital velocity in terms of actual speed instead of it’s slow .5º per hour angular velocity. At that point the math is very easy (though approximated). At a distance of roughly 240,000 miles the moon travels about 1.5 million miles to complete an orbit around the Earth. That’s roughly 55,000 miles per day or 2285 mph. At the equator the earth’s rotation is a bit more than 1000 mph, so the much faster eastward motion of the moon is (now) obviously the more important factor.

    • Admin says:

      Hi – While your first paragraph SEEMS intuitive, you are correct in the second paragraph by correcting yourself. The heliocentric perspective is the accurate one, as the Sun is casting the shadow! From the Sun’s perspective, in those three-odd hours that the Moon’s shadow is crossing the face of the Earth, its shadow does indeed cover the entire face of the Earth! (the angles work out, because the Sun is SO far away!). So, the Earth’s rotation counteracts the Moon’s shadow’s speed a bit, but there is only about three hours worth of time for it to have an effect – and that’s only 1/8 of a day – 45 degrees worth of Earth rotation! I think your second paragraph says it better. The math could be much more precise by invoking some trig, but essentially it’s a better way to look at it.

      If you have the math background, please check out the resources you’ll find in the bibilography of Meeus’ Canon of solar eclipses!


  6. Berta says:

    Ok but why does the sun appear rising over the Pacific Ocean–in the West?

    • Admin says:

      Where do you see that? In the Pacific Ocean, in the path, the eclipsed Sun will rise in the East as it always does. Don’t confuse where the Sun is in the sky from a certain location, to what the Moon’s shadow is doing. This animation by NASA shows it perfectly:


      • Marissa says:

        I’m not gonna lie and say I’m a math whiz or even that I’m excellent with directional cues, but this STILL doesn’t make sense to me. I’ve even made a little paper model here at home for myself. In the animation from the link you posted, it appears that the moon switches course….and it appears the globe is spinning in the wrong direction. Am I crazy? Is this an optical thing? What am I missing?

        • Admin says:

          It is an optical effect of the frame rate of the animation. Similar to when you used to see old movies, where the wheels of cars appeared to be rotating backwards due to the frame rate of the movie and the rotational speed of the wheel. The only thing that changes is the camera angle – the Moon continues to revolve in the same direction.

  7. Cheech says:

    So why does the animation STOP the earth’s rotation in this? This does not answer the question… The other day NASA had an animation that had the earth spinning the WRONG direction now they have the earth stand still!?!?

    • Admin says:

      Hi – I don’t understand the problem. The only places I see the animation pausing are so they can change camera angles. That isn’t quite the same thing as implying that the Moon ACTUALLY stops in its orbit!


  8. bx says:

    but i don’t understand. The earth is rotating 27 times faster than the moon rotating around the earth. That makes it impossible for the shadow to go from west to east. the earth is spinning around its access 27 times before the moon makes a full orbit around the earth. The shadow the moon creates between the earth and the sun would always travel from east to west. Impossible any other way.

    • Admin says:

      Sorry, but the Earth’s rotation compared to the Moon’s speed in orbit is only about half. Very round numbers here: The Earth rotates 27 times in a month (diameter of 8000 miles gives a total linear distance for a point on the surface at the equator of about…) 700,000 miles. The Moon moves (distance from earth = 240,000 miles gives an orbital circumference of about…) 1.5M miles in that same 27 days. So the Moon is actually moving a bit over twice as fast as a point on the Earth is moving. And points not on the equator are moving slower than that.

      You are trying to compare two things that are not really related, and the eclipse calculations take lots of things into account. But there’s the rough math of your argument. Eclipse calculations are something that are very well-known, and are simply correct – as you’ll see on eclipse day. The animation is correct.


  9. Admin says:

    Hello all – This thread is closed. No more posts will be entertained, as the discussion has now been taken over (behind the scenes – and thankfully we are shielding you all from that) by trolls. This is very sad. Please do this if you are still not convinced (this is called an “experiment”): Find two people who are in the path – one in Oregon, and one in South Carolina. Call them and have them text you when the eclipse is total for them. Then see how the eclipse is total in Oregon BEFORE it is total in SC. That would be proof that the eclipse moved from West to East, right? (or do we have to prove to you that Oregon lies to the west of South Carolina?) Then come back and see how many people who doubted this have corrected themselves… I think you will find nothing but crickets.

    Please, everyone, encourage your kids to take more science and math classes! This is the only way out! And there is still hope!



  10. Tony Tyner says:

    Thank you for this discussion. I’m 60 years old and a fan of astronomy. I “knew” all the facts about the moon and it’s rotation etc. but couldn’t understand why the moon was “backing up” onto the sun during the eclipse. (and all the animations prior. It turns out that I never really had a full understanding of the moons orbit. Because it “rises” in the east and “Sets” in the rest I was assuming East to West motion all these years even thought it was dead wrong.

    I am a very visual learner and those NASA videos are the bomb.

    I get it now that although the moon is “rising” in the east and setting in the west it’s orbit is moving the other way.

    I am really thankful for this understanding and feel like I finally “get” the moon, it’s rotation and the phases. etc.



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