Monday, 9 May 2016

Sic Transit Mercurius Monday

This afternoon, there was a transit of Mercury.  That is, from our perspective the planet Mercury passed in front of the Sun.  This happens at seven, thirty-three and thirteen year intervals, in either May or November, and takes up to three hours.  If I remember correctly, and I might well not, the May transits are better than the November ones because during the May ones Mercury looks bigger because it's closer.  Mercury is on one occasion ten arc seconds across and on the other twelve.  Apparently people don't know what arc seconds are, so here's an explanation.  A circle is divided into three hundred and sixty degrees.  Each degree is divided into sixty minutes of arc and each minute of arc divided into sixty seconds of arc.  If you consider the three hundred and sixty degrees of the horizon and the sky from horizon to horizon to be a hundred and eighty degrees, the smallest non-luminous object which can be seen is one minute of arc across.  Mercury is up to twelve seconds across, so it would need to be magnified five times to be visible even as a dot crossing the Sun,

I've never knowingly seen Mercury.  During the transit of Venus on 8th June 2004, I managed to project the Sun onto a piece of paper and it was quite clear.  Transits of Venus are much rarer than those of Mercury because Venus, being further from the Sun than Mercury, takes longer to orbit it and so passes between us and the Sun less often.  They occur in pairs a few years apart separated by a century or so.  This sounds odd at first until you realise that in order for Venus to pass in front of the disc of the Sun it has to do so in the plane of Earth's orbit.  Normally Venus passes by the Sun on either side, but if it is passing from "below" or "above" through the equator of the Sun it will be clearly outlined.  It also has the "black drop" effect, where Venus looks like a drop of black liquid as it passes the limb of the Sun because of its atmosphere, which is extremely dense.  At ground level, the atmosphere of Venus is about as thick as the water a whole kilometre down in the ocean here on Earth.  I found the transit of Venus to be particularly interesting because Venus is about the same size as Earth and so it gives you an idea of the scale of the Solar System.

Mercury does this much more often, but has no substantial atmosphere and is only a little larger than Cynthia as well as being further away than Venus on the whole, so although its transits are much more frequent they're harder to see.  This is what I got today when I was trying to project the transit of Mercury:

I've posted this as a full size image and you still can't see Mercury on it.  I don't think I was in the right state of mind to image it.  In other words, it didn't work.

What you need to do is get either a refracting telescope or, as in this case, a pair of binoculars and place a plain white surface at the appropriate distance to get a clearly focussed image.  This is a little risky since you're focussing sunlight and it can break prisms and lenses and cause fires.  However, I think people worry too much about this since as a child I used to filter the Sun through overexposed negative photographic film and look directly at it through binoculars and I can still kind of see.  I have multiple blind spots but probably not because of this activity.

What I like about transits, solar eclipses and looking at the sun safely from a home ed perspective is that they're daylight astronomy.  Two big problems with astronomy and home ed are that if you have a bedtime, you generally don't get to do much of it at night and, as a consequence of this, it tends to get rather abstract.  For this reason I didn't do a lot of astronomy with the children when I was doing Big Science and I'm not keen although it and linguistics are probably my personal two favourite subjects.

This is how the planets concerned were lined up this afternoon at about 2 pm:

As you can see, the Sun, Mercury and Earth are in a straight line.  This happens to be true seen from the side as well:

The next question might be why Mercury looks bigger in May, i.e. why is it closer.  This is because of Kepler's First Law of Planetary Motion.  Planets move in elliptical orbits with the Sun at one focus, i,e. the Sun is never at the exact centre of an orbit because that would be too improbable, and orbits are never circular because that also would be too improbable.  The orbit of Mercury is also more elliptical than any other true planet, but less so than that of Pluto.  Here are the orbits of Mercury and Earth side by side:

The Sun is clearly off to one side in the left hand picture and Mercury also clearly has a much less circular orbit.

This is a close up of Mercury:

Seen from even closer, even experts can't distinguish between it and Cynthia although it lacks the seas of the latter.  The craters, however, look very similar.

In fact, although both of them are barren cratered balls of grey rock, Mercury actually has quite a lot in common with Earth and Mars rather than Cynthia.  Mercury and Earth are the densest planets with Venus a close second, whereas Mars and Cynthia are both only about two-thirds as dense as Earth, Venus and Mercury.  It's been suggested that the reason for this is that Mars and Cynthia are formed from the debris of the outer layers of the original body making Earth up and that we are in fact the bits that sunk to the bottom.  Mercury is a bit bigger than Cynthia but quite a bit smaller than Mars, the figures being:

Cynthia:  3476 km diameter.
Mercury:  4879 km
Mars:  6792 km
Earth:  12756 km.

Even though Mercury is quite a bit smaller than Mars, its higher density gives it a surface gravity about the same.  This is interesting because it means that if Mercury were to be at about the same distance from the Sun as Earth is, it would probably have an atmosphere about as dense as that of Mars and also be roughly the same temperature as we are, notwithstanding a weaker greenhouse effect.  However, the air would probably be somewhat thinner than the Martian air because it has a lower escape velocity, as smaller planets have, so gases leak away into space more easily.  This is because an object doesn't have to go as far to orbit the planet, so a gas particle doesn't either and is more likely to be lost.  Even so, if Mercury was at the same distance as Earth is from the Sun, its sky would be blue and have occasional clouds in it and it might even have a little liquid water on its surface.  However, it wouldn't really have seasons because unlike Earth and Mars it doesn't really tilt.  What it might have if its orbit were still the same shape is the same seasons in both northern and southern hemispheres.  It also has a fairly strong magnetic field partly due to having a large iron core, which would protect it from radiation somewhat, and at a greater distance from the Sun it would have a shorter day (its day currently lasts 56 of ours) because of the weaker tidal forces.  All of this, rather surprisingly, adds up to Mercury being a more friendly place for life in this scenario than Mars is.

Mercury, ironically, probably has no Mercury on its surface since during the day it's hotter than its boiling point.  Strangely, however, Mercury has water ice at one pole, in the crater Chao Meng-Fu, because it's in permanent shadow.

I was going to say a lot more about Mercury but it's past my bedtime, so goodnight!