Astronomy would be easier but less interesting if the Earth stood still. In fact, the Earth has several different motions which astronomers long ago worked hard to understand. One of these motions is the Earth's rotation on its axis; another is the Earth's revolution, or orbital motion, around the Sun.
Reading: Stars & Planets, p. 14 — 16 (Star positions); p. 18 — 19 (Appearance of the sky), p. 21 (The Star Charts).
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, known as the north celestial pole, which just happens to be near the star Polaris. This is due to the Earth's rotation. As the spinning 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 move across the sky much like the stars.
Ancient astronomers explained this by supposing that the Sun, Moon, planets, and stars were all 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 when talking about the positions of stars.
The celestial pole is 21.3° above the horizon as seen from Oahu. The point on the horizon directly below the celestial pole is north, while the opposite direction is south. If you face north, east is on your right and west is on your left. Finally, the zenith is the point exactly overhead.
The north celestial pole is exactly overhead the Earth's north pole. Likewise, every point on the celestial equator is exactly overhead a corresponding point on the Earth's equator.
Once a year, the Earth makes a complete orbit about the Sun. As a result, the Sun appears to move with respect to the stars, passing 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. The planets also appear to move along the ecliptic, although, as we will see, they don't always move in the same direction as the Sun.
The night sky is just that part of the sky which 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 September, for example, the evening sky is still dominated by summer constellations like Cygnus and Sagittarius; by December, these constellations will be low in the western sky, and winter constellations like Taurus and Orion will be rising in the east. You can get a `sneak preview' of the winter sky by staying up late, thanks to the Earth's rotation. For example, the constellations visible at 8 pm in early December can also be seen at 2 am in early September.
The Earth's axis of rotation is not parallel to its axis of revolution; the angle between them is 23.5°. As a result, the ecliptic is tilted 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 revolving counter-clockwise about the Sun and rotating counter-clockwise on its axis. The other planets would likelwise revolve counter-clockwise around the Sun, and most would also 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. In the notes, this will be called `Hawaii Standard Time' and indicated by writing HST. For example, our class starts at 19:00 HST (= 7:00 pm + 12:00), and ends at 22:00 HST (= 10:00 pm + 12:00). Sometimes we need to record the date and the time together; for example, our first class began at 25-Aug-08 at 19:00 HST.
Astronomers all over 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 08-Sep-08 at 19:00 HST, or 09-Sep-08 at 05:00 UT. To convert from UT to Hawaii time, subtract 10 hours; if the result is less than 0, add 24 and go to the previous day. For example, we can see a stellar occultation (the eclipse of a star by the Moon) on 07-Oct-08 at 05:05 UT; that's 06-Oct-08 at 19:05 HST, just after the start of class.
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 hard to show how the sky really looks using a flat piece of paper, so reading an all-sky chart and relating it to what you see in the sky takes practice. 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. For some purposes, however, it's very convenient to show the entire sky in one chart, so you should learn to read these charts. All-sky charts for each month appear in Stars & Planets, starting on p. 24.
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. In your mind, stretch the chart so that it forms a dome over your head. The positions of stars on this imaginary dome now match their positions in the sky.
You can get a pretty good idea of how the sky will look on 09-Sep-08 at 20:30 HST by using the chart shown in Fig. 1. For example, the constellation of Cygnus (the swan), which is outlined in red, appears near the center of the chart, so it will be roughly overhead. Next to Cygnus is the bright star Vega; it is very near but just slightly above the center of the chart, so it will be seen very near the zenith but just slightly to the north. The planet Jupiter (indicated by the largest dot) and the Moon appear near each other, about half-way between the center of the chart and the southern compass point; they will appear in front of you, abouf half-way between the zenith and the horizon, if you face south. The bright star Acrturus, known to Polynesian navigators as Hokule'a (`star of gladness'), is seen toward the west, having passed almost directly overhead a few hours earlier.
If you are used to reading maps of the Earth, you may notice that the east and west compass points in Fig. 1 are reversed. On a terrestrial map with north at the top, you would expect to find west to the left and east to the right. However, a celestial map with north at the top has west at the right and east at the left. The reason for this is that a terrestrial map shows a view looking down at the Earth, while a celestial map shows a view looking up at the sky. Astronomical charts usually have north at the top and west to the right. When using a telescope, you'll notice that stars drift toward the west as a result of the Earth's rotation; this makes it easy to determine the correct orientation of a star chart.
|Fig. 1. The sky over Honolulu on 08-Sep-08 at 20:30 HST (09-Sep-08 at 06:30 UT), produced using Stellarium. Stars are shown as dots, with larger dots for brighter stars; the lines between stars trace constellations, with the constellation Cygnus highlighted in red. The blue curve is the celestial equator, and the red curve is the ecliptic. Compass points are shown around the edge of the chart.|
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 over 24 hours on 22-Sep-08 (the fall equinox). This illustrates the effect of the Earth's rotation.
Shows how the sky above Honolulu at 20:30 HT changes during one year, from 21-Dec-07 to 21-Dec-08. This animation illustrates the effect of the Earth's revolution around the Sun. Note how the constellations visible in the night sky change as the Earth revolves around the Sun. Also, note the Moon's monthly passages across the sky along the ecliptic and the fairly gradual motion of other planets.
Joshua E. Barnes
(barnes at ifa.hawaii.edu)
25 August 2008