Modern Astronomy

Lecture 2

August 28, 2002

Watch the skies! Watch the skies! If you have looked in the western sky in the early evening, you might have noticed a very bright object. That is the planet Venus, second planet from the Sun. Over the next few days, it will be at maximum easterly elongation, meaning it is at the largest angular distance it will get from the Sun. In about 1494, Copernicus figured out that this tells us how far Venus is in au. Watch Saturday night, when Venus will be very close to the bright star Spica, the brightest star in the constellation Virgo.

The Planets

Last time I mentioned that the major planets are in orbits around the Sun, just like the Earth is. There are a lot of other important schemes in the planetary properties. I mention these now because we will come back to them when we talk about other stars. The terrestrial planets are Mercury, Venus, the Earth, and Mars. They are (relatively) small, made of rock (and iron in the case of the Earth and Mercury), and are in the inner part of the solar system. What does inner solar system mean: the average distances of these planets from the Sun are 0.387,0.723,1.0, and 1.523 au.

The other four major planets are the Jovian planets of Jupiter, Saturn, Uranus, and Neptune. They are big (Jupiter, the biggest, is 10 times the diameter of the Earth and has 318 times the mass), are big gas balls, and are in the outer solar system. Their average distances from the Sun are 5.2, 9.5, 19.2, and 30.1 astronomical units. It is generally believed that there is a strong connection between where these planets are and what their properties are.

Nonetheless, the explanation of this connection is not quite so clear as it was ten years ago.

Jupiter and Saturn are morning objects, meaning you see them between about 2AM and dawn. Ditto with Mars. Uranus and Neptune are in the constellation of Capricornus, in the southeast in the early evening. You can see them with binoculars if you know where to look.

► If you want to look at a “road map” of the solar system, showing the relative distances of the planets, look at Figure 12.2.

► Incredibly enough, human-made spacecraft are even further out in the solar system than the major planets. Four spacecraft, Pioneer 11 and 12 and Voyager 1 and 2, are between 60 and 80 astronomical units from the Sun. All four had instruments built at the University of Iowa, and we still have communication with 3 of them.
Picture of Voyager Spacecraft

The Night Sky

One of the things I like to stress in astronomy courses is the connection between what we study in the textbook, and what is up in the sky. Throughout this semester I will make reference to certain stars as examples of red supergiants, or extreme old “Population II Stars”

(like Arcturus, the bright red star in the western sky these evenings) or the Andromeda galaxy (in great position for seeing this autumn).

First, let’s get our bearings. If you go out and stand in the Iowa countryside, it looks like you are in the middle of a plane (geometric term). If you want to describe the position of something in the sky, you can give its direction (north, east, south, west) and its angle above the horizon. The two angles you use to specify the position are the azimuth (north=0 degrees, east 90 degrees, south 180 degrees), and the altitude (right on the horizon =0 degrees, straight overhead is 90 degrees). This system of reference is called the horizon system.

► See Figure 1.14 for an illustration of this.

In the horizon system, we see objects in the sky do a number of things.

· The Sun rises in the east, reaches its highest altitude angle when it is in the south, and sets in the west.

· When the Sun sets it gets dark and we see the stars, the Moon, and the planets.

· The Moon rises in the east, reaches its highest altitude angle when it is due south, and sets in the west.

· The Moon rises at a different time every night, and appears against a different set of stars, or constellation.

· Here’s one that many people don’t realize. If you look at the constellations one hour after sunset (or one hour before sunrise), you see a different set every season. There is a slight night-to-night change, in that the stars seem to shift slowly to the west, so that after a few months you get a whole different set of stars in the night sky. This is illustrated in the star charts in the front and back of your textbook. This important phenomenon was well known to everyone prior to the invention of television and street lights. There are abundant references to it in Greek plays and Roman poetry.

► Question for the audience: What is going on to cause this east-to-west motion of all the celestial objects, this rising in the east and setting in the west?

● Next let’s talk about the seasonal differences in the appearance of the sky. If we go out and look at the sky at 9 PM tonight, we will see Scorpius and Sagittarius in the south, the star Altair in the southeast, and Vega straight overhead. If we do this in February at 9PM, we see Orion and the bright star Sirius (brightest star in the sky) in the south, and the bright yellow star (same color as the Sun) Capella overhead. What is causing this change?

The answer is the orbital motion of the Earth around the Sun. We don’t see stars in the same direction as the Sun.

► Illustration with diagram, demonstration.