General Astronomy - Miscellaneous


Where can I get astronomy posters, slides or videos?

You can get galaxy posters, as well as many other astronomy slides, posters, videos etc, from the Astronomical Society of the Pacific. Here is a link to their online catalog:
What would happen if you pour water into space?
Water poured into space (outside of a spacecraft) would eventually vaporize due to the very low density of matter in outer space.
What U.S. city had the first planetarium in the western hemisphere?
The Adler planetarium in Chicago, Illinois was the first planetarium built in the western hemisphere. It was constructed in 1930.
What are these lights we see in the night sky?
On a clear summer night, we look up and we see the sky aglow with starlight. All stars are actually Suns, and they come in all brightnesses from very bright to very faint - as do planets. But all lights in the sky however are not stars, of course. There are about 7 other (non-manmade) types of "lights" in the sky... Galaxies*, Globular Clusters*, Open Clusters, Emission Nebulae (like Orion's belt's central star, Comets, Planets, and Moons to see. (*require binoculars or a scope).
I saw the Leonids last night. Why did many meteors appear to be going in random directions?
First of all, a meteor shower usually does vaguely seem to come from a particular direction in the sky in general. First let's explain this and then go into why the rule is not perfect. It can be understood by understanding the origin of a meteor shower. When a comet in a closed orbit goes around the Sun repeatedly for many millennia, it can be broken into pieces by the stress of gravity. To some extent these particles will spread out over the orbit, filling the orbit and making something like a tube or stream of particles orbiting the Sun. If the orbit of Earth crosses the particle stream, the particles approach the Earth from generally the same distant direction (or vanishing point) and burn up. This direction is called the radiant. This also explains why meteor showers come regularly each year when the Earth is back in the same position crossing the stream and why they can last days (the time needed to cross wide streams).

Now, why don't all the particles always come from the same direction? You might guess that it's because some particles are moving in a direction not exactly aligned with the stream. But in fact they are traveling in parallel. Almost all the wacky directions seen in sky for the meteor trails the are projection effects...)

If we had the ideal situation, where the stream of particles were very confined and collimated, then the meteor trails would appear to radiate from a vanishing point in Leo (the "radiant"), much like railroad tracks converge in the distance to a point. If you were to look up the tunnel at the distant cometary debris from anywhere on earth, the tunnel would have a vanishing point in the sky at a point within constellation Leo.

But the Leonid stream is significantly wider than the diameter of the Earth. (This is the key fact). The meteors burn up in the outer shell of the upper atmosphere (50-150 km up) throughout the hemisphere facing upstream. But it's not like these particles are streaming at us from a single point in space, but more like they make up a big river (containing swarms and ribbon shaped groups of particles) which the sphere of the Earth is crossing. The Earth in its orbit around the Sun is going 11 km/s and its orbit takes it parallel to and in a mainly upstream direction with respect to the comet debris trail, (i.e. the Earth crosses very slowly, taking days to cross while moving upstream), and the particles are raining toward us at their own 60 km/s speed. These velocities add. (The rotation of the Earth on its axis adds a bit more to the impact after midnight.)

Particles can hit the upstream-facing Earth atmospheric hemisphere at any point, in the observer's North, South, East, or West, and from any elevation, from the horizon to the zenith, so long as that part of the sky is part of the hemisphere facing upstream. They fly in along strait paths, in general, and can burn up at different altitudes. The projection of the paths tends to point to the stream vanishing-point in Leo, but this is very approximate. Even though they are moving in the same stream, particles that hit the atmosphere at our distant horizon may look like they are traveling straight down. Particles that hit the atmosphere at a point over our local horizon may look like they are traveling up in the sky.

There is also some randomness to the particles motion coming into the atmosphere (but not much relative to the collision speed). The particles seen burning up last night were first boiled off the main comet in the 1770's as it went around the Sun, and they still have some left over random velocities. Also, some paths might be distorted within the atmosphere (the atmosphere can cause some particles to skip, like a rock over the water). However, the main cause of different apparent directions for the meteors in the sky is the fact that they entering the curved atmosphere at different points and lighting up at different heights and we are visualizing them projected onto a simple flat sky. If you could watch from a point removed from the Earth, it would look like a straight driving rain.

Isn't it something the way the coordinates of objects keep progressing to the East (or is it to the West?) and is that due to precession?
There are three precessions acting on the Earth orbit. That is due to the precession of the equinoxes, second, the precession of the perihelion, and finally the relativistic precession "anomaly."

1) The precession of the equinoxes is dominant. It is caused by the Moon (2/3) and the Sun (1/3).

The Earth is in the funny condition of being tilted at 23 1/2 degrees to the plane of the Solar System (or the ecliptic). As viewed from way above the North Pole, Earth rotates on it's axis counterclockwise (CCW), and orbits the Sun CCW in a nearly circular ellipse. But as with any spinning top acted on by an outside force, it also nutates - its head wobbles in a circle.

If you imagined and arrow sticking out of the North Pole being watched from way above the Solar System, by an observer fixed (relative, say, to the Sun) the pole arrow would be seen to rotate in a circle Clockwise (!) once every 24,000 years.

Why "CW?"

You see, a normal top, spun on the floor, acted on by the Earth gravity seeking to topple it down, nutates in the same direction as it spins. The best analogy to a top is the glass stopper to an olive oil carafe. Sure enough, when spun, the cork on the top of the stopper circles in the same direction as it spins. (You will also notice a spun quarter nutating forward as in make the last part of its descent onto a table.)

The reason for the nutation at all is because, for a spinning object, the downward force vectors express themselves perpendicular to the plane made by the spin axis and the downward force via Newton's Laws.

Then, why does the Earth axis nutate in the sky in a direction opposite its spin direction?

It is due to the fact that the force on the Earth is caused by the Sun and Moon, and this force is trying to erect the Earth spin axis perpendicular to the ecliptic plane. I.e. the Moon and Sun would like to stand the Earth up on end to be like most other planets (which would cease the occurrence of seasons). Were the Earth perfectly spherical, there would be no net tipping force caused by the Sun and the Moon (just leaving the force to stretch Earth and in turn to bring up tides). But since Earth is oblate (the equatorial diameter is 27 km greater than the polar diameter) the Sun and Moon have something to deal with - love handles. And, were in not spinning, it would be set erect very quickly.

There is a simple diagram that shows why the forces of ecliptic objects erect oblate spheroids, but I will spare you.) But since it's spinning and the Moon and Sun forces try, but don't succeed to erect it - instead, the forces cause nutation in a direction opposite its spin. I.e., we keep our 23 1/2 degree inclination, we keep our seasons, but Polaris is only once in a while the Pole Star.

Since its nutation is CW when looked at from above, it's CCW when looked at from the ground. (Just as East is counter intuitively to the left when you lay on your back with your head pointing North).

Another way to say this is if you lay on you back for 24,000 years worth of nights, you would see the stars circle a point in the sky which itself would complete one 23 1/2 degree radius CCW circle.

Since the position of the "fixed" stars is determined from their displacement from the point where the projection of the Earth equator into space intersects the ecliptic plane in space; the Equinoxes, their positions precess. These displacement distances are called "Right Assentions, or RA's."

Another way to see this is as follows. Imagine half the sky. Imagine the fixed stars as dots on a hemispherical bowl. The rim of the bowl is in the plane of the ecliptic. Imagine a _tilted_ ring that just fits into the rim of the bowl. This is the Earth equator extended into space. The ring nutates (wobbles around its center) CW as viewed from above and so then only two points of the ring always touch the rim of the bowl. These are the equinoxes. Since the ring nutates CW viewed from above the two equinoxes travel CW as viewed from above once each 24,000 years. The distance from one of these equinoxes to a dot (fixed star) increases in time (50 arc sec/year) with the distance measured to the left (East) increasing, as viewed from below. Thus, the RA's of objects grow 50" year to year due to precession. There is a declination change too, but it's just due to the fact that we are dealing with elliptical and equatorial systems that are not aligned.

2) There is another precession - the precession of the perihelion.

This is easier to understand. It is due to the fact that the elliptical Earth orbit-shape itself is tugged around the Sun by the perturbation of the other CCW (as seen from above) orbiting planets. We are tugged-at a little and the Earth orbital ellipse, as seen from above, itself orbits the Sun every 140,000 years (10 arc sec/year). This is called the precession of the perihelion (shortest vector between Earth and Sun) which now happens on Jan 2 - the shortest day). This actually does not subtract from the CW (as seen from above) precession (1) since it tracks the Earth path position (which still stays nearly circular) and not our fiducial measurement points, the Equinoxes.

3) There is a tiny fractional arc sec/year precession "anomaly" due to relativity.

Half of it is explained by the Special Theory of Relativity (1/3 due to time dilation and 1/6 due to mass increase) over the fast portion of our orbit (we move faster and get more massive in January this millennium). The other half requires the General Theory of Relativity to explain and I am not going to try because it's over my head these days.

An interesting story connected with the Relativity Anomaly (that you may know) is that the precession of the Perihelion of Mercury (the understood part was about 5000 arc sec/century) had an unexplained 43 arc sec per century till Einstein explained it with GR, and it is the one of the three or four proofs of GR.