Tomorrow’s Total Lunar Eclipse, and a Mystery

January 19th, 2019 by Roy W. Spencer, Ph. D.

Tomorrow night (January 20-21) will present the whole U.S. with a total lunar eclipse, the best one until May 15, 2022.

Totality here in Alabama will occur approximately from 10:40 to 11:40 p.m. CST. Clear weather will be restricted mostly to the southeastern U.S., and portions of the Northern Plains and Great Lakes:

A Mystery (to me, anyway)

There’s one aspect of the eclipse I cannot figure out. I’m sure the explanation will be simple, and when someone explains it to me, my response will be, “DOH!”.

The illumination of the moon during totality is due to light scattered through Earth’s atmosphere. Just as we see red sunsets, that red light will be shining on the moon from an annulus of red sunset light circling the Earth.

What I don’t understand, though, is the role of sunlight refraction (bending of sunlight) as it passes through the atmosphere at an oblique angle. The refraction occurs whether it is the moon or the sun being viewed through the limb, and I will use the example of moonlight shining through the limb.

My understanding is that light (from either the moon or sun) bends as I crudely show in the following cartoon. The “mystery” arises from the fact that we know that the appearance of the moon is that it is flattened due to refraction (this is NOT a diagram of what is happening during the eclipse.. it’s a general question about how either sunlight or moonlight is refracted as it passes close to Earth’s limb):

The moon composite photo is from the ISS, so it is exactly analogous to the situation shown in the drawing.

So, the mystery: Why is the moon flattened rather elongated? I simply don’t know. But I’m sure the explanation is simple.

Update: Mystery Solved

As I suspected, the problem was in the way I was looking at it. As
Brent Auvermann suggests in the comments, here’s the proper way to look at it. The eye sees the top and bottom of the moon at 2 slightly different angles, which are normally separated by 0.5 deg. But when the view in the direction of the bottom of the moon (0.5 deg. below the top of the moon) goes through a lot of atmosphere, it gets refracted downward, and the view from that direction comes from below the moon. In other words, what is a 0.5 degree subtended angle viewed by the eye actually originates from a bigger angle than that on the other side of the Earth’s limb. That causes the bottom of the moon to be compressed into a smaller angle (flattened):

42 Responses to “Tomorrow’s Total Lunar Eclipse, and a Mystery”

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  1. Ric Werme says:

    I just posted this on your FB page:

    Good point. Something else for me to be obnoxiously pedantic about!

    I suppose when someone wanted a throwaway, sciency explanation of the color, refraction didn’t raise many questions. OTOH, scattering might conjure images of grade school science of scattering in water glasses with a couple drops of milk or dust or fog.

    From the moon, the only refraction to be seen comes from the bottom 10-20 miles of Earth’s atmosphere, i.e., essentially nothing. On the other hand, the red light comes from Rayleigh scattering away the green and blue portions of sunlight in that narrow layer, then the remaining red light has to change direction to reach the moon during totality because the sun is blocked by the Earth. (Rayleigh scattering is what makes the sky blue.)

    When we see red sky after the sun goes below the horizon, that comes from forward scattering of sunlight and reflection off clouds. I think we could see the latter from the moon, there ought to be some good conditions in the 24,000 miles that could source that. Refraction is not involved!

    We need a term that triggers people to think of the sunrise and sunset ring around the Earth, but I haven’t come up with one yet. What we really need is a good time lapse video from the “edge” of the moon of a total eclipse. It would have the lunar horizon with the Earth hanging just above and show how the red ring changes during the sun’s “travel” behind the Earth.

    • Bart says:

      I think the first graphic simply has the light bending the wrong way.

      We are used to seeing the image of the light bent towards the Earth for an observer on the Earth, so that e.g., the Sun can be seen from below the horizon. But, this is a manifestation of Fermat’s principle, which says that the rays take the path between the source and observer such that transit time is minimized.

      It is very important to specify where that observer is. For the Earthbound observer, the atmosphere slows the rays down, so the rays take a path that arches up out of the atmosphere and back down into it, minimizing the time spent going through it.

      For the extraterrestrial observer in Dr. Spencer’s first diagram, the rays will want to bend up, away from the atmosphere, to reach the observer in the least time.

  2. Ric Werme says:

    When someone (US? China? Musk?) finally posts that time lapse from the moon, I think millions of people will suddenly realize we could have, should have done that decades ago.

    It’s a pity that Apollo 15 wasn’t able to get a good photo (and they might not have seen totality anyway).

    There’s this, but I’m not interested in a lunar diamond ring effect, I want to see the sunrise/sunset ring in red!

  3. Ric Werme says:

    Oh, as for why the moon is flattened, I understand why, but don’t have good drawing tools.

    If you ray trace things from the viewer to the sun, the lower traces will be higher than they would be if the planet wasn’t in the way. That will make the moon look squashed.

    Perhaps easier to describe: We know geometrically (sans refraction), the lower limb of the sun is below the horizon. We know refraction is letting us see it above the horizon. We know the upper limb isn’t refracted as severely. Therefore what we see must look as though the lower and upper limbs are closer than geometry shows.

    • look at my diagram and tell me what’s wrong with it. It shows what I’ve found for refraction diagrams, but those diagrams would be consistent with an elongated (oval), moon, rather than a flattened moon. I must be missing something.

      • Brent Auvermann says:

        No, your diagram is consistent with a flattened moon. If you trace the LOS of the lower edge of the moon scene straight away from your eye, without bending it (which is the way your eye perceives the scene; it doesn’t care whether the light has bent on the way or not), that LOS will intersect the ACTUAL moon partway up from the bottom arc.

        • Brent Auvermann says:

          Your diagram needs to be adjusted a bit. Because at the time in question the eye is at the far end of the LOWER ray, the top “ray” should be drawn so as to be parallel to the the bent end of the lower ray, not horizontally as you’ve got it drawn right now. That will show how the scene compresses in the vertical.

          • Brent Auvermann says:

            Another way to correct your diagram would be to draw the rays from right to left, that is, from the eye as the vertex to the top and bottom of the moon. First draw the lower ray as if there were no refraction (no intervening body), then draw the lower ray to the bottom of the moon WITH refraction. The change in scene observed by the eye will result from the difference in vertex angle at the eye. Your diagram assumes that the two light rays observed by the eye are emitted by the moon parallel to one another, but that is not the case.

          • Bart says:

            I don’t think that’s it. See above.

      • Ric Werme says:

        Your diagram is accurate for an observer who is deformed vertically as totality begins. His eye gets stretched so it can see both the upper and lower light rays that you drew. 🙂

        • Terry says:

          While the compression in the vertical is explained, why then is it not also compressed in the horizontal plane as well to the same extent. Just curious

      • Bart says:

        To further elucidate my comment above, the light is not going to follow a path that makes it dip into the atmosphere more than it would for a straight line. That would extend the time of transit, rather than reducing it.

  4. DocSiders says:

    During the eclipse we are looking at the moon…”using” light coming FROM the moon…it doesn’t matter how the light got to the moon (except for the color reddening from Rayleigh scattering)…so the “lensing” of light from the sun from atmospheric bending is not involved with the light we “use” to see the moon.

    The apparent size of the moon should be the same as any “near eclipse” full moon.

  5. CO2isLife says:

    Dr. Spencer, the moon not only flattens it turns red. That is a clue. Think about what is happening to the spectrum that is reaching your eye as the moon approaches the horizon. The blue light gets cut off first and then Green and then yellow etc etc. Red light, the light that gets bent the least in refraction is what you are seeing. Red light is basically horizontally compressing the image relative to the other wavelengths. That is my best guess.

    • Ric Werme says:

      No, if the atmosphere had a high dispersion, then the images of the moon from the ISS would have significant color fringing above and below them. Instead they have (remarkably?) little.

      The red is from the same effect that we see in sunrise and sunsets – Rayleigh scattering has scattered the blue and some green creating our blue sky, leaving longer wavelengths.

  6. CO2isLife says:

    Oooops, look at the atmosphere, it is like a convex lens. The shape of the atmosphere is acting like a lens and distorting the image. Take a magnifying lens and look through it at an angle. That is what is happening.

    • JDHuffman says:

      With only a few minutes of thought, I’ll go with refraction, similar to a magnifying glass.

      Look through a magnifying glass at an object about 3 feet away. Tilt the magnifying glass toward the object and the object compresses vertically. Rotate the glass, and the object compresses horizontally.

      It’s all I can do to avoid silly flat-earth jokes about a “flat-moon”….

  7. Bart T says:

    Draw two parallel lines intersecting near the top (12 noon) of a circle. The distance between the lines is small, compared to the radius of the circle.

    Note the angle of incidence of the bottom line is some value less than, but close to, 90 degrees. Say, it is 85 degrees. Then, the angle of incidence of the top line is closer to 90 degrees. Say, it is around 87 degrees perhaps.

    Now, you calculate the angle of refraction with Snell’s law. You can use an on-line calculator to see that the level of refraction increases as the angle of incidence approaches 90 degrees.

    Could this be the answer to your question?

  8. Bart T says:

    One other thing, regarding your drawing. It would be more correct to show both top and bottom lines from the moon as being apparent to an observer on the earth.

    Then, with the top line being refracted slightly more, the ISS image seems logical.

  9. CO2isLife says:

    You are observing an optical distortion called positive coma. It is what happen when light hits a lens at an angle and the focus gets distorted.

  10. Entropic man says:

    The refraction effect compresses the angle suntended by the moon.

    Away from the horizon the moon subtends half a degree, 30 minutes.The upper limb subtends 15 mnutes and the lower limb 15 minutes.

    As the moon sinks lower, the denser air closest to the surface refracts and compresses the bottom of the image more than the top. The width is still 30 minutes, but the depth decreases, with the lower limbdecreasing faster than the upper limb.

    In the last of the images the upper limb subtends 7 minutes and the lower limb 4 minutes.

    The upper and lower rims of the image each trace half an ellipse. The centre of the lower limb has much less curvature than the upper limb, so the lower limb appears flat.

  11. ren says:

    The graphic of the polar vortex shows the current circulation well. You can see that strong frost will attack Eastern North America.

  12. .
    ❶①❶① . . . The recent Slowdown – on trial . . .

    Alarmists have started a legal battle, in an effort to convict the recent Slowdown of a serious crime. The crime in question is, “impersonating a real Slowdown”. This heinous crime carries a maximum sentence of 20 years of watching Al Gore “documentaries”.

    The trial is about to begin. We have managed to get our “climate reporter”, Sheldon Walker, on to the jury hearing the case against the recent Slowdown. We asked Sheldon if he thought that it was “fair”, for him to be on the jury? Sheldon replied, “Is it “fair”, that Alarmists won’t admit that there was a small, temporary Slowdown, that doesn’t have any significant long-term implications for global warming”?

    Sheldon is prepared to go to extreme lengths to help his friend. He has taught himself to text message with his toes, using a cellphone that is hidden in his shoe. Sheldon will be sending us text message “reports” from inside the room where the jury members are deliberating. These text message reports will be limited to 160 character per text message (Sheldon refuses to use Twitter), so Sheldon will use abbreviations where necessary.

  13. ren says:

    The stratosphere forecast points to even more frost in the east of the US and in Europe.

  14. DocSiders says:

    What does viewing the moon from outer space have to do with viewing a lunar eclipse?

  15. CO2isLife says:

    Dr Spencer, your graphic shows the moon being enlarged, not crunched. The original image is correct. This is the correct image.×309.jpg

    This image is incorrect.×309.jpg

    For the image to flatten, you have to have the rays diverging as they approach the observer. You then trace the rays back to their focal point to get the “virtual image.” You have two kinds of optical distortions.

    1) Chromatic aberration
    2) Positive Coma

    Light bends towards the higher index of refraction material. In this case, the atmosphere has a higher index of refraction than outer space. The bending of light towards the earth projects back as if the Moon is being crushed.

  16. Ken says:

    Does anyone know where to find out the last time there was a super blood wolf moon on 20 January? If climate is a cycle based on gravity and magnetics … this would be a bench mark.

  17. ren says:

    This is what the stratosphere intrusion looks like in the east of North America.

  18. ren says:

    The current temperature (C) in the east of the US.

  19. Jerry Hudson says:

    Prof. Spencer,

    I agree with your revised explanation, only I would draw the diagram slightly differently. The “flared out” ray bundle should encompass the entire Sun or Moon. The dotted line which is tangent to the lower, more highly curved ray, then strikes the object somewhere in the middle. That of course is where the lower limb appears to be, squashing the object in the vertical direction.

    A little off-topic is the differential refraction of light, which often causes the upper rim of the Sun to become detached and colored bright green, even sometimes blue. An excellent book on this topic is _The Green Flash and Other Low Sun Phenomena_ by Father O’Connell of the Vatican Observatory.

    Saw the eclipse thru a break in the clouds last night – my granddaughters were spellbound.

    – Jerry Hudson

  20. Mike Flynn says:

    Kudos to Dr Spencer.

    Even the optics minuscule visible component of the EM spectrum can create questions – even to intelligent and well qualified individuals. Few have the courage to admit their lack of knowledge, and seek answers.

    Amazing that many supposed experts blithely assume that they know all about invisible light, from radio waves to x-rays and beyond, and their interaction with matter of various types and constitutions.

    As an example, the frequencies emitted by a domestic microwave oven heat water nicely, have almost no effect on microwave safe plastic, ceramics, etc., and spectacular effects on most metal objects.

    What about IR of 2000 – 5000 nm? Lenses of germanium are opaque to visible light, but refract and disperse the IR.

    And so it goes. Not everyone who talks about radiation necessarily understands it.


  21. ren says:

    It is worth seeing how the polar vortex is broken in the middle of the stratosphere.
    The temperature felt in New York reached yesterday -29 degrees C.

  22. Howard Walter says:

    Your first drawing was almost accurate, but you show the bending AFTER the light passes thru the atmosphere.

    If you draw it bending IN the Atmosphere., its easy to see that the bottom limb of the moon appears higher, making it appear flatter.

  23. Walter Allensworth says:

    Thanks for the flattened moon explanation Dr. Spencer.
    I hope to learn something new every day, and today it was early!

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