Celestial bodies

There is no real difference between an average star and the sun: both art gigantic glowing gas spheres in infinite space, emitting a huge quantity of light. As our distance from the stars is so great, we merely see them as tiny luminous points, which by day are outshone by the light of our nearest star, the sun. The sun emits unpolarized light, and so also do the stars. On its long way to us, however, starlight meets many particles of dust, which under the influence of magnetic fields, have a preferential orientation in space. As a result, the starlight reaches the Earth more or less Polarized, but this polarization is so low (at most a few percent) that it cannot be observed with simple devices.

Unlike the sun and the stars, the moon, planets and comets do not themselves emit light but reflect sunlight to us. In these cases, linear polarization is very likely especially when the reflection occurs at about an angle of 90º , the celestial body being then in its first or last quarter. However, only the moon, Mercury, Venus, the comets and artificial satellites are capable of this. The outer planets (Mars, Jupiter, Saturn etc.) always appear to us to be almost 'full', and for this reason show hardly any polarization - again only a few percent or less, and therefore not visible with simple devices.

The surface of the moon consists or rough stones and dust. Its polarization is not nearly so strong as that expected from a smooth reflecting surface. The maximum degree of polarization occurs when the moon is slightly more than half-lit: averaged over the whole lunar disc, however, it is no more than about 10 %. The direction of polarization is tangential with respect to the sun, hence directed 'from above downwards'. It is difficult to see the polarization, because the moon is such a small object. Dark areas are more polarized than light ones (the Umov rule) and the degree of polarization may amount to 20 % or more for the maria, which is comparable to that of terrestrial sand-plains. Indeed, the polarization is visible with careful scrutiny.

The Earth-shine of the moon (the illumination of the dark section of the moon) also proves to be faintly polarized. Here as well, the polarization is maximal in the first and last quarters, when the light is rather dim. The maximum degree of polarization is also about 10 %. The Earth-shine comes from the Earth with its oceans, clouds and plains. The polarization of the light of the planet Earth as a whole can be considerable (about 40 % maximum), but after reflection against the dust-covered moon this light loses a large part of its polarization.

In favourable circumstances, comets can have a degree of polarization of 30 % or more. Most of the light of a comet comes from its tail, and here, too, it is really sunlight that is scattered to us by gas molecules. Some artificial satellites can also be strongly polarized, to 40 % or more. It is, however, not so easy to observe the polarization of these rather faint, rapidly moving objects. The degree of polarization of artificial satellites depends on the materials of which they are constructed, and there are many satellites that have hardly any polarization.

The polarization of Mercury is about the same as that of the moon: maximum about 10 %,. The structures of their surfaces are comparable. However, it is even harder to observe the polarization of Mercury, with its much fainter light, than that of the moon, indeed almost impossible for amateurs. Venus with its dense atmosphere is virtually unpolarized: in fact, its degree of polarization is even lower than that of Mercury.

Apart from what has already been mentioned, there is little else in the nocturnal sky which shows any polarization. Some gas clouds in space light up, because they scatter light coming from a nearby star, and this light is polarized. Yet the degree of polarization is rather low (usually about 20 %) and extremely difficult to observe because of the faintness of their light. An example of such a cloud is VY CMa in the constellation of the Greater Dog.

An interesting challenge, however, is the polarization of the Crab Nebula M1 in Taurus. It is a remnant of a supernova explosion which took place in 1054; the then exploded star is now the famous pulsar which is in the nebula. The light of the Crab Nebula is synchroton radiation, which is locally highly polarized (to about 70 %!). The direction of polarization depends upon that of the local magnetic fields, and it can vary greatly from place to place. The strongest polarization is found at the edges of the nebula, where, however, the light intensity is less than at the centre. The shape of the nebula changes when we watch it through a rotating polarizing filter. A big telescope is needed for observing this effect, and the polarizing filter must be put in front of the mirror because otherwise the polarization may be changed by reflections in the optical equipment. The observation is not easy to make but worth while because of the way in which the polarization is displayed. Together with the solar corona and comets it is the only astronomical object with a really, strong polarization.