Smallest- electron

neutron & proton

ion & atom & element & isotope

Largest- molecule

proton

neutron

electron

The method used here is spectroscopy. Using samples of light coming from a star we can determine which gases are present through absorption lines in the spectrum. These lines represent places where a wavelength has been absorbed before reaching the detector. Since different elements absorb different wavelengths, we can tell exactly what is in the star’s atmosphere. The higher the abundance, the more the absorption.

1 Period = 24-25 days, depending on the latitude; its gas, so does not all spin at the same rate.

This is a common astronomical problem because all distances are hard to measure. The one word answer is PARALLAX. The position of the moon relative to the stars depends on your position. The moon is near enough that you can make two measurements, at the same time, form different places on the Earth. If you know the change in position of the moon and the distance between the places of observation, then the distance to the moon comes from geometry. Its like Fig 2.1 in the book, except that B and A can both be on the Earth at one time.

Because the moon is near, we can use radar. The time for any form of electromagnetic radiation to travel to the moon and back = 2d/c, where d is the distance to the moon, and c the speed of light.

Radar is practical within the inner solar system.

Note that it is not enough to measure the angle subtended by the moon, which can be done as follows. Hold up a coin between your line of sight and the moon, far enough so that it just covers the moon completely. Then measure the distance between you and the coin. Now you can set up a ratio between the diameter (twice the radius) of the coin (d) and the diameter of the moon (D) and set it equal to the ratio between the distance measured (s) and the distance to the moon (S). See picture below:

The problem here is that we find the ratio d/s, which is the same as D/S, but we still do not know D because we do not know S.

This planet has a density between that of Saturn (0.7 g cm^-3), and Uranus (1.2 g cm^-3). So it would be composed of hydrogen, helium, and some ices.

If the mass of a planet is not very large, its gravitational field will not be great enough to hold a light gas like hydrogen in its atmosphere. Hydrogen moves faster than the escape velocities of these planets.

Two possible ways to answer this. The first is better.

1. Luminosity is defined as the intrinsic brightness, or energy output. The moon does not release energy, including light, hence its luminosity is zero.
2. The moon does reflect sunlight, so we might consider this as the moon’s luminosity. This is a much more complex answer, since a calculation is needed, and it is beyond the normal scope of this course. We would start with the luminosity of the sun, 4 x 10²⁶ Watts, and calculate the fraction of that which falls on the moon. This fraction is the fraction of the sky occupied by the moon, as seen form the sun. This fraction is given by the area of the moon’s disk: 3.14 x (1738 km)² = 10 million km², divided by the surface area of the sphere at the Earth’s orbit: 3 x 10¹⁷ km² (nearly all of the light leaving the sun will pass through this sphere). The moon then intercepts 3x10^-11 of the sunlight. We need to reduce the result because the moon reflects only 11% of the light which falls on it (this is the moon’s albedo: top p. 68). The result is 4 x 10²⁶ x 0.11 x 3x10^-11=10¹⁵ W. While small compared to the sun, this is still a huge number. We do not get fried by moon light because the moon is far away.