Before we can start star-gazing, we need to define some basic directions and terminology, as well as a system for measuring time. This section also introduces the charts astronomers use to represent the entire night sky.
Background Reading: Stars & Planets, p. 13 & 14 (Star positions); p. 16 & 17 (Appearance of the sky).
Astronomy would be pretty easy, and pretty dull, if the Earth didn't move. In fact, the Earth has several different motions which must be understood in order to do astronomy. One of these motions is the Earth's rotation on its axis; another is the Earth's revolution, or orbital motion, about the Sun.
If you watch the night sky for a few hours, you will see that the stars appear to rotate about a fixed point in the sky (which happens to be near the pole star, Polaris). This motion is due to the Earth's rotation. As the spin of the Earth carries us eastward at almost one thousand miles per hour, we see stars rising in the East, passing overhead, and setting in the West. The Sun, Moon, and planets appear to move across the sky much like the stars.
Because of the Earth's rotation, everything in the sky seems to move together, turning once around us every 24 hours. Ancient astronomers explained this phenomenon by supposing that the Sun, Moon, planets, and stars were attached to a huge celestial sphere, centered on the Earth, which rotated on a fixed axis once per day. Of course, this sphere does not really exist; the Sun, Moon, planets, and stars all fall freely through space, and only appear to move together because of the Earth's rotation. Nonetheless, we still use the concept of the celestial sphere in talking about the positions of stars.
The celestial sphere appears to rotate around a fixed point, the North celestial pole, which is 21.3° above the horizon as seen from Oahu. The point on the horizon directly below the celestial pole is due North, while the opposite direction is South. If you face North, East is on your left and West is on your right. Finally, the Zenith is the point exactly overhead.
Since the apparent rotation of the celestial sphere is due to the actual rotation of the Earth, the North celestial pole is exactly overhead as seen from Earth's North Pole. Likewise, every point on the celestial equator is exactly overhead from some point on the Earth's equator.
Over the course of a year, the Earth makes one complete orbit about the Sun. As a result, the Sun seems to move with respect to the stars, appearing in front of one constellation after another, as shown in the diagram on p. 12 of Stars & Planets. After one year, the Sun is back where it started. The Sun's annual path across the sky is called the ecliptic. Traditionally, the ecliptic was divided into twelve equal parts, each associated with a different constellation of the zodiac.
The night-time sky is just the part of the sky that we see when the islands of Hawaii have turned away from the Sun. As we orbit the Sun, different constellations are visible at different times of the year. In January, for example, the evening sky is dominated by Winter constellations like Orion and Taurus; by the end of the semester, these constellations will appear low in the western sky, and Spring constellations like Leo and Centaurus will be visible instead.
The Earth's axis of rotation is not precisely parallel to its axis of revolution; the angle between them is 23.5°. Consequently, the ecliptic is inclined by the same angle of 23.5° with respect to the celestial equator. This misalignment causes seasons; when the Sun appears North of the celestial equator the Earth's northern hemisphere receives more sunlight, while when the Sun appears South of the celestial equator the northern hemisphere receives less sunlight.
If we could view the Solar System from a point far above the North Pole, we'd see the Earth rotating counter-clockwise on its axis and revolving counter-clockwise about the Sun. Most of the other planets would also appear to rotate counter-clockwise. In addition, the Moon would appear to orbit the Earth in a counter-clockwise direction, as would most other planetary satellites.
In this class, we will use a 24-hour clock instead of writing `am' or `pm'. Since our class meets in the evening, most of the times we will record are after noon, and the 24-hour time is the time on your watch plus 12 hours. For example, our class starts at 19:00 (= 7:00 pm + 12:00), and ends at 22:00 (= 10:00 pm + 12:00). Sometimes we need to record the date and the time together; for example, our first class begins at 01/14/03, 19:00.
Astronomers all over the world use a single time system to coordinate their observations. This system is called Universal Time, abbreviated as UT or UTC. (Greenwich Mean Time, abbreviated GMT, is the same thing as UT.) Universal Time is exactly 10 hours ahead of Hawaii time. To convert 24-hour Hawaii time to UT, you add 10 hours; if the result is more than 24, subtract 24 and go to the next day. For example, our first observing session (weather permitting) will be at 01/21/03, 19:00, or 01/22/03, 05:00 UT. To convert from UT to Hawaii time, you subtract 10 hours; if the result is less than 0, you add 24 and go to the previous day. For example, Mercury will pass in front of the Sun from 05/07/03, 05:13 UT to 05/07/03, 10:32 UT; that's 05/06/03, 19:13 to 05/07/03, 00:32 in our time zone. (Unfortunately, the Sun will have set here on Oahu before this interesting event begins!)
As a rule, we will use 24-hour Hawaii time in this class, and write the time without any time zone. The `UT' symbol will be used only when we want to specify universal time.
Astronomers represent the appearance of the entire sky as seen at some particular place and time by drawing circular all-sky charts. Unfortunately, it's not really possible to capture the appearance of the sky on a flat piece of paper, so reading an all-sky chart and relating it to what you see in the sky is a bit tricky. For example, these charts distort the patterns of stars near the horizon, so you may find it hard to recognize constellations from an all-sky chart. The only way to correct this distortion is to break the sky up into several separate charts (this is the approach used in The Sky Tonight, which we will use to find the constellations). For some purposes, however, it's very convenient to show the entire sky in one chart, so you should learn to read an all-sky chart.
To read an all-sky chart, hold it in front of you with the side labeled `N' at the top. Now imagine you are lying flat on your back with your head pointing North; then East will be on your left, South at your feet, West on your right, and the Zenith right in front of you. Mentally stretch the disk of the chart so that it forms a dome over your position. The positions of stars on this imaginary dome now correspond to their positions in the sky.
|The sky over Honolulu on 01/21/03, 21:00, produced using Your Sky. Stars are shown as dots, with larger dots for brighter stars; the connecting lines show constellations. The symbols show the positions of Jupiter and Saturn. The blue curve is the celestial equator, and the red curve is the ecliptic. The red `x' is used in a review question below. Compass points are shown around the edge of the chart.|
If you try this exercise with the chart shown here, you can get a pretty good idea of how the sky will look on 01/21/03, 21:00. For example, Saturn is near the center of chart, so it will be almost exactly overhead. Jupiter is on the left side of the chart, about one-third of the distance from the edge to the center, so it will be visible in the East, about one-third of the way from the horizon to the Zeneth. The North Star, Polaris, is near the top of the chart, so it will be visible in the North; the Little Dipper hangs down toward the horizon from Polaris, and it will only be visible if we have a good view toward the North (which Kapiolani park, alas, does not).
Note: this is an advanced topic. We won't have much use for celestial coordinates in this class, but you'll see them mentioned from time to time.
Just as latitude and longitude can be used to specify any point on the Earth's surface, two celestial coordinates can be used to specify any point on the celestial sphere. Imagine starting from the point on the sky the point where the Sun, moving North, crosses the celestial equator (this is the point labeled `0 h' in the chart above). To reach any given point on the celestial sphere, you could first travel along the celestial equator, and then towards one of the celestial poles, until you reach your destination. The angle you've traveled along the equator is called the right ascension; it's measured in units of hours, where 1 hour = 15°. The angle you've traveled towards one of the poles is called the declination; it's measured in degrees, with positive declinations towards the North celestial pole, and negative declinations towards the South celestial pole.
As already noted, celestial coordinates won't be used much in this class. They're included here because they are used in Stars & Planets. Typically, the book gives celestial coordinates when discussing stars; for example, if you look at the description of alpha Orionis on p. 194, you'll see `5h 55m +7°.4' just after the star's name. This means that alpha Orionis has a right ascension of 5h 55m (just slightly less than 6 hours) and a declination of +7°.4. Celestial coordinates also appear on the constellation charts; for example, see the chart of Orion on p. 195, which shows that Orion lies across the celestial equator at about 5h 30m right ascension.
An interactive planetarium, set up to show the sky now above Honolulu. You can chose other dates and times, select other viewing sites, and zoom in on selected areas; for these and other options, see http://www.fourmilab.to/yoursky. Created by John Walker.
Shows how the sky above Honolulu changes during one day, from 01/01/03, 21:00 to 01/02/03, 21:00. This animation illustrates the effect of the Earth's rotation. Note: the various symbols you see moving along with the stars represent the Sun, Moon, and planets. On this particular day, the Moon appears close to the Sun, although it's not actually close enough to cause an eclipse!
Shows how the sky above Honolulu changes during one year, from 01/01/03, 21:00 to 12/31/03, 21:00. This animation illustrates the effect of the Earth's revolution around the Sun.
Last modified: January 14, 2003