Tag Archives: sun

Is it Meaningful to Say that Earth Goes Round the Sun, or Not? (And Why Is This So Hard…?)

Is the statement “The Sun Orbits the Earth” false? Not according to professor Richard Muller of the University of California, Berkeley, as I discussed yesterday. Muller argues that Einstein’s theory of general relativity implies that you can view the Sun as orbiting the Earth if you like, or that both the Sun and Earth orbit Venus, or a random point in space, or anything else for that matter. Meanwhile, every science textbook in our kids’ classrooms says that “The Earth Orbits the Sun“. But for all of our discussions yesterday on this subject, we did not yet collectively come to any conclusions as to whether Muller is right or wrong. And we can’t hope to find evidence that the Earth orbits the Sun if the reverse is equally true!

When we’re trying to figure out whether a confusing statement is really true or not, we have to speak precisely. Up to this stage, I haven’t been careful enough, and in this post, I’m going to try to improve upon that. There are a few small but significant points of clarification to make first. Then we’ll look in detail at what it means to “change coordinates” in such a way that would put the Sun in orbit around the Earth, instead of the other way round.

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Sun Around the Earth, or Earth Around the Sun?  Did Einstein Say “It’s all the same”?

We’re all taught in school that the Earth goes round the Sun.  But if you look around on the internet, you will find websites that say something quite different. There you will find the argument that Einstein’s great insights imply otherwise — that in fact the statements “The Earth goes round the Sun” and “The Sun goes round the Earth” are equally true, or equally false, or equally meaningless.

Here, for example, is this statement as written in Forbes by professor Richard Muller at the University of California, Berkeley.   It opens as follows: “According to the general theory of relativity, the Sun does orbit the Earth. And the Earth orbits the Sun.”  I invite you to read the rest of it; it’s not long.

What’s his point?  In Einstein’s theory of gravity (“general relativity”), time and three-dimensional space combine together to form a four-dimensional shape, called “space-time”, which is complex and curved.  And in general relativity, you can choose whatever coordinates you want on this space-time. 

So you are perfectly free to choose a set of coordinates, according to this point of view, in which the Earth is at the center of the solar system.  In these coordinates, the Earth does not move, and the Sun goes round the Earth.  The heliocentric picture of the planets and the Sun merely represents the simplest choice of coordinates; but there’s nothing wrong with choosing something else, as you like. 

This is very much like saying that to use latitude and longitude on the Earth is just a choice. I could use whatever coordinates I want.  The equator is special in the latitude-longitude system, since it lies at latitude=0; the poles are special too, at latitude +90 degrees and -90 degrees. But I could just as well choose a coordinate system in which the equator and poles don’t look special at all.

And so, after Einstein, the whole Copernican question — “is the solar system geocentric or heliocentric?” — is a complete red herring… much ado about nothing. As Muller argues in his article, “the revolution of Copernicus was actually a revolution in finding a simpler way to depict the motion, not a more correct way.

Well? Is this true? If not, why not? Comments are open.

Earth Goes Around the Sun? What’s Your Best Evidence?

It’s commonly taught in school that the Earth orbits the Sun. So what? The unique strength of science is that it’s more than mere received wisdom from the past, taught to us by our elders.  If some “fact” in science is really true, we can check it ourselves. Recently I’ve shown you how to verify, in just over a dozen steps, the basics of planetary astronomy; you can

But important unanswered questions remain.  Perhaps the most glaring is this: Does the Earth orbit the Sun, or is it the other way around?  Or do they orbit each other around a central point?  The Sun’s motion in the sky relative to the stars, which exhibits a yearly cycle, indicates (when combined with evidence that the stars are, on yearly time scales, fixed) that one of these three must be true, at least roughly.  But which one is it?

We saw that the Earth satisfies Kepler’s law for objects orbiting the Sun; meanwhile the Sun does not satisfy the similar law for objects orbiting the Earth.  This argues that Earth orbits the Sun due to the latter’s gravity, but the logic is circumstantial. Isn’t there something more direct, more obvious or intuitive, that we can appeal to? 

I won’t count high-precision telescopic observations that can reveal tiny effects, such as stellar aberration, stellar parallax, and Doppler shifts in light from other stars.  They’re great, but very tough for non-experts to verify. Isn’t there a simpler source of evidence for this very basic claim about nature — something we can personally check?

Your thoughts? Comments are open. [Be careful, when making suggestions, that you are not assuming that gravity is the dominant force between the Earth and the Sun. That’s something you have to prove. Are you sure there are no additional forces pinning the Earth in place, and/or keeping the Sun in motion around the Earth? What’s your evidence that they’re absent?]

Which is Bigger, the Sun or the Earth?  Check it Yourself!

Once you’ve convinced yourself the Earth’s a spinning sphere of diameter about 8000 miles (13000 km), and you’ve estimated the Moon’s size and distance (diameter about 1/4 Earth’s, and distance about 30 times Earth’s diameter), it’s easy to convince yourself the Sun’s bigger than the Earth, and much further than the Moon.  It just takes a couple of triangles, and a bit of Moon-gazing.

Since that’s all there is to it, you can guess that the ancient Greek astronomers, masters of geometry, already knew the Sun’s the larger of the two.  That said, they never did quite figure out how big and far the Sun actually is; we need modern methods for that.

It’s Just a Phase

The Moon goes through a monthly cycle of phases, lasting about 291/2 Earth days, in which the part that glows brightly with reflected sunlight grows and shrinks, from crescent to full and back again.  The phases arise because there are two simple ways of dividing the Moon in half:

  • At any moment, the half of the Moon that faces Earth — let’s call it the near half of the Moon — is the only half that we can potentially see. (We’d only be able to see the far half, facing away from Earth, if the Moon were transparent, or a big mirror was sitting beyond the Moon.)
  • At any moment, the half of the Moon that faces the Sun is brightly lit — let’s call it the lit half.  The other half is dark, and its presence can only be detected by the fact that it can block stars that it moves in front of, and through a very dim glow in which it reflects sunlight that first reflected from the Earth (called “Earthshine.”)  

The phases arise because the lit half and the near half aren’t the same, and the relationship between them changes from night to night.   See the diagram below. When the Moon is more or less between the Sun and the Earth (it rarely passes exactly between, because its orbit is tilted by a few degrees out of the plane of the drawing below) then the Moon’s lit half is its far half, and the near half is unlit. We call this dark view of the Moon the “New Moon” because it is traditionally viewed as the start of the Moon’s monthly cycle. 

Figure 1: The Moon’s phases, assuming the Sun’s much further than the Moon. When the Moon is roughly between the Earth and Sun, its near half coincides with the unlit half, making it invisible (New Moon). As the cycle proceeds, more of the near half intersects with the lit half; after 1/4 or the cycle, the Moon’s near half is half lit and half unlit, giving us a “half Moon.” At the cycle’s midpoint, the near side coincides with the lit half and the Moon appears full. The cycle then reverses, with the other half Moon occurring after 3/4 of the cycle.

When the Moon is on the opposite side of the Earth from the Sun (but again, rarely eclipsed by Earth’s shadow because of its tilted orbit), then its near side is its lit side, and that creates the “Full Moon”, a complete white disk in the sky. 

At any other time, the near side of the Moon is partly lit and partly unlit. When the line between the Moon and Earth is perpendicular to the Earth-Sun line, then the lit side and unlit side slice the near side in half, and the Moon appears as a half-disk cut down the middle.

When I was a child, I wondered why half this half-lit phase of the Moon, midway between New Moon (invisible) and Full Moon (the bright full disk), was called “First Quarter”, when in fact the Moon at that time is half lit.  Why not “First Half?”  Two weeks later, the other half of the near-side of the Moon is lit, and why is that called “Third Quarter” and not, say, “Other Half”?

This turns out to have been an excellent question. The fact that a Half Moon is also a First Quarter Moon tells us that the Sun is large and far away!

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An Experience of a Lifetime: My 1999 Eclipse Adventure

Back in 1999 I saw a total solar eclipse in Europe, and it was a life-altering experience.  I wrote about it back then, but was never entirely happy with the article.  This week I’ve revised it.  It could still benefit from some editing and revision (comments welcome), but I think it’s now a good read.  It’s full of intellectual observations, but there are powerful emotions too.

If you’re interested, you can read it as a pdf, or just scroll down.



A Luminescent Darkness: My 1999 Eclipse Adventure

© Matt Strassler 1999

After two years of dreaming, two months of planning, and two hours of packing, I drove to John F. Kennedy airport, took the shuttle to the Air France terminal, and checked in.  I was brimming with excitement. In three days time, with a bit of luck, I would witness one the great spectacles that a human being can experience: a complete, utter and total eclipse of the Sun. Continue reading

A Solar Eclipse Tomorrow (Sunday)

Appropriate for General Readership

Tomorrow there will be a solar eclipse (i.e., the moon will pass between the earth and the sun, blocking the sun’s light.) Those of us on the east coast of the United States who wake up to a clear sky at dawn will see the rising sun partially eclipsed, as much as half blocked in many places. [Don’t forget that in the US the clocks are changing tonight, so dawn is one hour earlier, as the clock tells it, than it was today; in New York City sunrise is at 6:30 am tomorrow.] Meanwhile, a substantial partial eclipse will be visible across most of Africa, and a less substantial one in parts of southern Europe.  And a little sliver of central Africa will be fortunate enough to see one of nature’s most extraordinary spectacles: a total eclipse of the sun, where for a couple of minutes the sky suddenly goes almost dark, the stars come into view, and the pink prominences and silvery corona of the sun glow and shimmer in the darkness of the moon’s shadow.

Really, this ought to have been scheduled for Halloween.  Because if you didn’t know to expect a total solar eclipse, and you didn’t know what was going on, there’d be nothing more terrifying.

Remember: Except in the truly dark heart of a total eclipse, looking at the sun for even a few moments can destroy your eyes; either use specially designed “eclipse glasses” (ordinary sunglasses are completely unsafe) or use a pinhole in a piece of cardboard to project the sun’s image onto a piece of paper or a wall. [As I described here, carefully placed binoculars pointed at a piece of paper or wall will work too — but do not look through them!!! just let the sun’s image go through.] For those watching at sunrise, if there is cloud or haze in the east that dims the sunlight, you can look for a few moments — but make it very quick!


A number of transitions to talk about:

First, I’m participating in a panel discussion today (Thursday June 7th) on the transition that has seen me add science writer and popularizer to my resume’. Here’s the link… free tickets required, click here for details and ticketsSponsored by SoNYC (Science online New York City), panel discussion entitled “Reaching out of the Ivory Tower”, about the experiences of scientists who are reaching out to the public. Panelists: Ethan Perlstein, Sarah Weisberg, Matt Strassler, Jeanne Garbarino.  Location and time: Weiss 305, Rockefeller University, East 66th and York Ave. New York, NY, 7:00 PM.   Presenting science to the public in a digestible but honest form is something I feel is important, and I’ll have a few words to say about why I chose to do it now and why via a website and blog. And then I guess the floor will be open to questions, so come on by and ask one!

Reminder: again in New York, Saturday June 16th at 2pm, I’ll be giving a lecture (click here for details):  THE EINSTEIN OBSESSION: SCIENCE, MYTH AND PUBLIC PERCEPTION.

Next: I hope you all enjoyed seeing Venus in transit across the Sun Tuesday, in whatever method you chose. If you missed it live, there are of course opportunities to see films of the event — and they’ll save you time, since they’re all speeded up so that the hours pass in a minute or so. Nice views of Venus are complemented by several good-sized sunspots that are roughly the size of the Earth. Venus, you may recall, is the same size as the Earth, but as it was only a third as far away as the Sun yesterday, it appeared three times larger in radius (10 times larger in surface radius) than those sunspots. I managed to see it with the naked eye (with eclipse glasses), but just barely… so tiny! (By the way, if you missed my article on how a transit of Venus was used to obtain the first high-precision measurement of the distance to the sun, here’s the link.)

Here’s one of my own best shots, which I show you not because you can’t find better shots on the internet but because I have learned that most people do not realize you can get such nice views of the sun with so little work.

Venus in transit, at bottom.  Several sunspots dot the central region of the sun’s disk.  Photo of projection through binoculars onto white screen.  Image has been darkened slightly to sharpen contrast between sun and background.  Photo: M. Strassler 2012, all rights reserved.

It’s really quite easy.  And though there won’t be any more transits of Venus for us to see, there will be bigger sunspots, transits of Mercury, and solar eclipses to watch over the coming 15 years, so you may as well learn how to do this. You just aim the binoculars, big side toward the sun (as though someone were going to look at the sun — BUT DON’T LET ANYONE LOOK, of course), in the rough direction of the sun, wiggle the binoculars until the shadow of the binoculars becomes as small as it possibly can (which tells you they are aligned with the sun) and at that point the sun will shine through the two sides of the binoculars, giving you two images. Put a lens cover on one of the sides if you want to just get one image. Then let the image project onto something white and smooth that can serve as a screen. By moving the screen forward or back you can get a larger or smaller image; by using the focus on the binoculars you can bring the image of the sun into focus. Works great!

Here’s a picture, taken in a similar way, from the annular eclipse last month — a transit of the moon, even though we don’t usually call it that — taken the same way. I like this shot (though focus is imperfect and it was taken from a funny angle) because where the moon’s shadow touches the sun’s limb you can see a bit of light shining between mountain ranges on the moon!

A few seconds before annularity begins during the annular eclipse of the sun on May 20, 2012. Note, in the region where the moon’s silhouette touches the sun’s limb, a patch of sunlight shining through the mountain ranges on the moon. Light and dark regions at left and right of photo are due to shadows on the projection screen.  Photo: M. Strassler 2012, all rights reserved.

Venus, passing between the Sun and the Earth on Tuesday, has now transitioned from an object in the evening sky to one in the morning sky; if you want to see it, large and a very thin crescent through binoculars, you’ll need to get up early, before sunrise.  Not yet, though; it will be a few days before Venus is far enough from the Sun to pick out in the dawn sky.  For now, you can see Venus via the SOHO satellite, which blots out the sun so what’s nearby can be observed.

A final transition: Ray Bradbury died. Bradbury was one of the 20th century’s great science fiction writers, and I especially enjoyed reading his stories because he didn’t write classic geeky science fiction. His work was much more thoughtful and human than that. I think one could make a case that what made his writing unique was that he didn’t separate science from the rest of life.  It’s a good example for the rest of us to follow.

Transit Day

I hope you’re all ready for today’s transit of Venus across the face of the Sun.  You certainly can’t have missed that it’s happening, given the media hype.  (Look at the website http://transitofvenus.nl/wp/, their post from yesterday in particular, for all sorts of useful resources, including timing for the event in various locations, and on-line resources for you to watch in case clouds interfere at your location. UPDATE: that website is overwhelmed. In the continental U.S. transit begins just after 6 pm Eastern, 3 pm Pacific, differing by a few minutes from place to place; and sunset will occur before the transit is over. One online location to watch the transit is http://events.slooh.com/)   But — let me be the first to warn you — this is going to be very cool, but it isn’t going to be a spectacular event like a big meteor shower or a total solar eclipse or even a total lunar eclipse.  It’s going to be subtle, slow, and potentially very boring, unless you have the right mindset (or a truly excellent telescope, properly filtered for sun viewing).  So here are some suggestions: Continue reading