No Comet, But Two Crescents

I’m sure you’ve all read in books that Venus is a planet that orbits the Sun and is closer to the Sun than is the Earth. But why learn from books what you can check for yourself?!?

[Note: If you missed Wednesday evening’s discussion of particle physics involving me, Sean Carroll and Alan Boyle, you can listen to it here.]

As many feared, Comet ISON didn’t survive its close visit to the Sun, so there’s no reason to get up at 6 in the morning to go looking for it. [You might want to look for dim but pretty Comet Lovejoy, however, barely visible to the naked eye from dark skies.] At 6 in the evening, however, there’s good reason to be looking in the western skies — the Moon (for the next few days) and Venus (for the next few weeks) are shining brightly there.  Right now Venus is about as bright as it ever gets during its cycle.

The very best way to look at them is with binoculars, or a small telescope.  Easily with the telescope, and less easily with binoculars (you’ll need steady hands and sharp eyes, so be patient) you should be able to see that it’s not just the Moon that forms a crescent right now: Venus does too!

If you watch Venus in your binoculars or telescope over the next few weeks, you’ll see proof, with your own eyes, that Venus, like the Earth, orbits the Sun, and it does so at a distance smaller than the distance from the Sun to Earth.

The proof is simple enough, and Galileo himself provided it, by pointing his rudimentary telescope at the Sun 400 years ago, and watching Venus carefully, week by week.  What he saw was this: that when Venus was in the evening sky (every few months it moves from the evening sky to the morning sky, and then back again; it’s never in both),

  • it was first rather dim, low in the sky at sunset, and nearly a disk, though a rather small one;
  • then it would grow bright, larger, high in the sky at sunset, and develop a half-moon and then a crescent shape;
  • and finally it would drop lower in the sky again at sunset, still rather bright, but now a thin crescent that was even larger from tip to tip than before.

The reason for this is illustrated in the figure below, taken from this post [which, although specific in some ways to the sky in February 2012, still has a number of general observations about the skies that apply at any time.]

A planet (such as Mercury or Venus) with an orbit that is smaller than Earth's has phases like the moon but grows and shrinks during its orbit round the sun due to its changing distance from earth.  It is always largest when a crescent and smallest when full, and is brightest somewhere in between.

A planet (such as Mercury or Venus) with an orbit that is smaller than Earth’s has phases like the Moon.  The portion of Venus that is lit is a half-sphere (shown in light color); the portion of Venus we can see is a different half-sphere (dashed red lines); the overlap is shaped like the wedge of an orange and looks like a crescent in the sky.  But unlike the Moon, which is at a nearly fixed distance from Earth, such a planet appears to grow and shrink during its orbit round the Sun, due to its changing distance from Earth. It is always largest when a crescent and smallest when full, and is brightest somewhere in between.

So go dig out those binoculars and telescopes, or use Venus as an excuse to buy new ones! Watch Venus, week by week, as it grows larger in the sky and becomes a thinner crescent, moving ever closer to the sunset horizon.  And a month from now the Moon, having made its orbit round the Earth, will return as a new crescent for you to admire.

Of course there’s another proof that Venus is closer to the Sun than Earth is: on very rare occasions Venus passes directly between the Earth and the Sun.  No more of those “transits” for a long time I’m afraid, but you can see pictures of last June’s transit here, and read about the great scientific value of such transits here

13 responses to “No Comet, But Two Crescents

  1. Marshall Eubanks

    The Comet ISON Observers meeting is being streamed live today (starting 8:30 AM EST Dec 6) on Livestream
    Although the discussion is at times technical, there is a lot of discussion about what the now famous photos of its perihelion passage mean.

    • Thanks — wish I could listen in. If you are listening and you hear anything surprising, could you please let us know? In particular, I’d love to know: why didn’t the SDO satellite see *anything*?

  2. Marshall Eubanks

    There was a SDO presentation about 9:30 EST – they feel that there must have been no significant evaporation of the dust. SDO looks at Extreme UV Oxygen lines, and comets have lots of Oxygen in Silica rocks, carbonaceous material, etc. They batched 1 hour worth of images at a time and saw nothing.

    (I have to wonder if the comet could have been _Iron_ plus water & other ices. Iron meteors have very little Oxygen.)

    An interesting thing was (finally) an explaination of the “L shaped” tail post-perihelion – it was dust emitted _before_ perihelion. If dust had been still being emitted, there would have been an anti-solar tail (pointing away from the Sun), which was not observed post-perihelion.

  3. I’ve been enjoying the views of Venus. It is fairly easy to see the crescent with binocs or a small telescope. I hope people try it.

  4. “If you watch Venus in your binoculars or telescope over the next few weeks, you’ll see proof, with your own eyes, that Venus, like the Earth, orbits the Sun, at a distance smaller than the distance from the Sun to Earth.”

    I had to read that sentence many times because how can the Earth orbit the sun at a distance smaller than the distance from the Sun to the Earth?

    “like the Earth” should be “like Mercury”

  5. Richard Benish

    The fact that Galileo perceived the likely correctness of your graphic of Venus orbiting the Sun is one of the reasons he is regarded as the father of modern science. Your recommendation to see for yourself instead of believing what is written in books further echoes the ideals of science.

    In this vein, I’d like to mention another observational idea proposed by Galileo which, however, has NEVER been carried out. Galileo pondered the result of dropping a cannonball into a hole through the center of Earth. Though impossible to carry out on such a large scale, the idea is nevertheless quite possible to carry out in a satellite or an Earth-based laboratory (with a modified Cavendish balance).

    Books tell us that, according to both Newton’s and Einstein’s theories of gravity, the dropped object harmonically oscillates in the hole. Karl Schwarzschild, who derived the most famous solution of Einstein’s field equations—the EXTERIOR solution for a spherical mass—also derived an INTERIOR solution. This represents the most ponderous half of the gravitational universe. But the solution has never been tested. Performing Galileo’s experiment would serve as a test.

    Why believe the book answer when we can find out what happens by the direct observation of Nature herself? All we need is an apparatus that may be called a Small Low-Energy Non-Collider.

    • Edwin Steiner

      Here is an experiment that is a little bit similar to what you describe:
      It is a measurement of the gravitational constant G where the test masses were suspended through holes in big field masses. It is quite interesting to read which experimental pains were taken by the authors to improve the accuracy. Still the obtained precision is nowhere close to the precision of measurements of other physical constants. The extreme weakness and the universality (i.e. no shielding possible) of gravity make it incredibly hard to measure. In fact the latest measurements of different groups are not in agreement. An interesting story, although the most probable explanation are unknown systematic errors.

  6. Richard Benish

    Thanks for the link.

    In the experiment described therein, the hole through the source masses functions more as a means of conveniently moving them with respect to the test masses than as a means of measuring gravity inside them. Some tests of the inverse square law involve measuring gravity inside source masses by delicate suspension mechanisms.

    Dozens or hundreds of gravity experiments have been done by measuring the tiny deviation of suspended masses compared to the lack of deviation when the source masses are removed.

    What has never been done is an experiment in which the dominant gravitational effect is revealed by allowing test masses to move freely through the center of source masses. That is the essence of the experiment proposed by Galileo.

    Note that the general relativity (GR) prediction (which may be derived from the Schwarzschild interior solution) can be thought of in terms of clock rate. The Newtonian oscillation prediction corresponds to the GR prediction that a clock at the center has a minimum rate. This evokes the question, why should a clock at the center of the source mass tick slow? What makes that happen, if it does? By symmetry, and by analogy with rotation, for example, one might expect the central clock to have a maximum rate (as a clock on the axis of a rotating body has a maximum rate).

    We could debate and compare analogies and predictions forever and never really know the answer until someone finally sees fit to do the experiment.

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  8. I read another prrof that venus orbits the sun

    The full set of Venereal phases can happen only if Venus passes both in front of and behind the Sun as seen from Earth (Figure 1, left). The Ptolemaic model placed Venus orbiting the Earth closer than the Sun, but always near to the Sun as constrained by observations, but that would preclude gibbous phases from being seen since that would require the Earth to be roughly between the Sun and Venus. On the other hand, moving Venus’ orbit beyond that of the Sun would allow gibbous phases, but would not permit crescent phases to be seen.

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