Astronomy would be easier, but far 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 with 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 are all falling through space, and only appear to move together because of our planet's rotation. Nonetheless, we still use the concept of the celestial sphere when talking about the positions of stars and other objects.
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 lies exactly overhead at 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 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 our part of the Earth is 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 begins at 23-Aug-2011 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 30-Aug-2011 at 19:00 HST, or 31-Aug-2011 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 (a star eclipsed by the Moon) on 30-Nov-2011 at 05:46 UT; that's 29-Nov-2011 at 19:46 HST, soon after class starts.
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; this makes it harder to recognize constellations. 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.
If you are used to reading maps of the Earth, you may notice that the east and west compass points on a sky chart are reversed. On a terrestrial map with north at the top, west is 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 things 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.
To use an all-sky chart, hold it in front of you with the side labeled `N' at the top. 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 in front of you. In your mind, stretch the chart to form a dome over your head. The positions of stars on this imaginary dome now match their positions in the sky.
|Fig. 1. The sky over Honolulu on 30-Aug-2011 at 20:30 HST (31-Aug-2011 at 06:30 UT), produced using Stellarium. Stars and planets are shown as dots, with larger dots showing brighter objects; the lines between stars trace constellations. The blue curve is the celestial equator, and the red curve is the ecliptic. The small blue cross shows the north celestial pole; compass points are shown around the edge of the chart.|
You can get a pretty good idea of how the sky will look on 30-Aug-2011 at 20:30 HST by using the chart shown in Fig. 1. For example, the bright star Vega appears near the center of the chart, so it will be near the zenith (overhead). The bright star Acrturus, known in Hawaii as Hokule'a (`star of joy'), having passed overhead a few hours earlier, is halfway between the zeneth and the western horizon. The planet Saturn (indicated by a medium-sized dot which is not part of a constellation) appears just above the western horizon. Maui's fish-hook, part of the constellation of Scorpius, shines in the south.
Shows how the sky above Honolulu changes over 24 hours on 22-Sep-2011 (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-2010 to 21-Dec-2011. This animation illustrates the effect of the Earth's motion around the Sun. Note how the constellations visible in the night sky change as the Earth moves around the Sun. Also, note the Moon's monthly passages along the ecliptic and the fairly gradual motion of other planets.
Joshua E. Barnes
(barnes at ifa.hawaii.edu)
20 August 2011