2. Understanding the Sky
Astronomy is full of cycles, such as the day, the year and the month. These are due to things like the rotation of the Earth, the orbit of Earth round Sun, orbit of moon round Earth and orbit of planets round Sun. In this section we will try to understand these cycles. We will be using computer software to demonstrate how the sky changes. Goals here are
I'm doing this at an early stage in the course because I want you to watch the sky over the next few months This is possibly the hardest part of course. It took civilization thousands of years to understand what is going on so you should not feel bad if it takes you a couple of weeks. There is a simple reason why this section is difficult. We are trying to understand the apparent motion of moving objects (such as planets) when we ourselves are also moving, albeit too slowly for our senses to detect directly. |
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2.1 The SkyDirect, Scattered and Reflected lightMost of what we see in everyday life is scattered light; exceptions are electric light bulbs, and the Sun which actually generate light rather than scatter it. In astronomy it is particularly important to distinguish between objects that generate light (eg the Sun and the stars) and objects which scatter (or reflect) light, such as the Moon and the planets. The sky is bright during the daytime because of scattering of light in the Earth's atmosphere. If you look at a photograph of astronauts on the moon you see that the sky is dark, even though the Sun is casting shadows. We exist in a Universe with stars all around us. The only reason we don't see them in the daytime is that the sky around them is too bright. With some kinds of telescope we can see stars even in broad daylight. The Earth's AtmosphereThe Earth's atmosphere does not have a sharp edge. It tails off gradually, so that the pressure is down to about half the sea level pressure at an altitude of 7 km. You can see a blue edge to the Earth in satellite pictures, such as this picture of the MIR spacecraft. At the summit of Mauna Kea you are above about 1/3 of the Earth's atmosphere. Space shuttle orbits at about 100 miles, which is small compared with the diameter of the Earth which is 8000 miles. In human terms this is almost like being in a vacuum, but there are still traces of the Earth's atmosphere even at this altitude. |
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2.2 Constellations, names and sky mapsMapping the skyConsider the Earth in the middle of a 3-dimensional forest of stars. For this section we can forget about the Sun, the Planets and the Moon, and ignore the rotation of the Earth. Just by glancing at the sky we don't know the distance to stars (to measure distances to stars we need to make very careful observations with a telescope; we'll learn about those later.). What we see is what looks like a pattern on the inside of a sphere. We call this imaginary sphere the "Celestial Sphere". We can make maps of the patterns of stars. From the surface of the earth we can never see more than half of the sky at a time, but we can piece together an atlas of the whole sky.There are problems in making maps of large regions of the sky on a flat sheet; these problems are the same as those that face geographers making maps of the curved surface of the Earth. If we stand in the desert or in the ocean we can see half of the universe; our "horizon" defines what we can and cannot see. The point directly above your head is called the "zenith" We also need to define an imaginary line in the sky that runs from north, through the zenith to due south. This is called the "meridian". The "altitude" of a star is the angle between the horizon and the star. |
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Constellations and star namesThe positions of stars relative to each other change only very slowly. (Although the positions of the stars relative to the horizon change by the hour, as we shall learn later) Ancient astronomers saw almost exactly the same patterns we do thousands of years ago. They called these patterns constellations, and often linked them with their mythology. Note that the stars in one constellation not necessarily at the same distance from the Earth, and are probably quite unrelated to each other in space. The whole sky is divided into constellations, just as the whole USA is divided into states Only very bright stars have names. In the West these name are often derived from Arabic names. Most stars are referred to either by number or by their position in the sky. Millions have numbers; billions more exist on photographs and have never been given names Many people enjoy learning and recognizing constellations, but it is not necessary for this course. |
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Angles on the skyIn describing the sky's appearance we need to discuss the angles between stars We can talk about one star being 10 degrees from another. This is nothing to do with how many miles or light years they are apart. We use degrees for large angles, for small angles we use arcminutes (1/60 of a degree) and arcseconds (1/60 of an arcminute) The Sun and Moon are both about a half a degree across. A penny held at arms length is about 1 degree (depending on your arm!). A shaka sign is about 20 degrees The coordinate system that astronomers usually use to describe the position of a star is rather like latitude and longitude but I do not need you to understand how to use it. Like latitude and longitude, the coordinates are based on angles. |
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2.3 Daily rotation of EarthFor this section we will continue to ignore the Sun, Moon and planets. The difference from the last section is that now we are considering the effects of the fact that the Earth rotates about its axis once per day. Rotation of the Earth means that stars appear to move. The motion can be seen with a time exposure, or speeded up with a computer. When a star crosses the meridian we call that a "transit" Point above the Earth’s North pole is the North Celestial Pole, very close to the location of the star Polaris. There is also a South Celestial Pole, but there is no particular "south star" at that position. Stars equidistant from the two celestial poles are on the celestial equator. [Run Starry night: show rising and setting, transit] Direction of rotationIf you look down onto the Earth from above the North Pole the Earth appears to rotate to counterclockwise. In other words it rotates towards the East. Stars therefore appear to move from East to West in the sky.
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Variation with LatitudeThe latitude of a place on Earth describes its North-South position. Hawaii has a latitude about 20º North. Lines of latitude run from side to side on a map. (If you confuse longitude and latitude remember "Latitude is Flatitude") If you stand at the Earth's north pole you will always see Polaris above you, (ignoring the brightness of the daytime sky) with the sky rotating around Polaris. The same stars are visible above your horizon all the time. No stars ever rise or set. If you stand at the Equator, Polaris will be on the horizon, and all stars will rise and set. In principle you can see all the stars in the Universe at some time or another, although only half will be visible at any time. The celestial equator runs directly overhead from east to west. If you stand at an intermediate northern latitude you will see Polaris at an angle above the horizon that is equal to your latitude (about 20 degrees in the case of Hawaii). Some constellations near Polaris are visible all through the night. They are called circumpolar. Some constellations near the south celestial pole are never visible. Most constellations can be seen some of the time as they rise in the East and set in the West It is (in principle) very easy for a navigator to determine his/her latitude on the Earth. All you need to do is measure the angle between Polaris and the horizon. If you can't see Polaris (in the southern hemisphere, for example) you can use other stars and make some calculations, but the principle is the same |
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Variation with LongitudeLongitude describes the East-West position of a place on the Earth's surface. London (actually Greenwich) is at longitude 0º, and Hawaii is at (roughly) longitude 157º west. Stars transit at different times at different longitudes on Earth. The same star crosses the meridian about 10 minutes later later in Honolulu than in Hilo and about 10.5 hours later in Honolulu than in London. If you can measure that time difference accurately (using, say, telescopes at each location) you can determine the longitude difference between the two places. Nowadays with radio, it is very easy to know the time in London, so you can determine longitude easily. You can also do this is you carry an accurate clock with you on the ship, but such a clock (known as a chronometer) was not invented until the 18th Century, about Captain Cook's time. Traditional Polynesian navigation (eg using the Hokulea) could not make use of a clock, so all sorts of clever tricks had to be used Measurement of timeHistorically our measurement of time was always based on the rotation of the Earth. Now that we have very accurate atomic clocks we find that there are small variations in the rotation speed of the Earth, probably due mainly to changes in the motions of the molten iron in the earth's core. Astronomers keep track of the rotation of the Earth and decide when we need to add (or subtract) an extra "leap second" into our clocks. Leap seconds (if needed) are usually added at the end of the year and are announced well in advance |
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2.4 Motion around the SunIn this section we consider the consequences of the Earth's revolution around the Sun once per year. We will come back later to the historical controversy of how we know that it is the Earth that moves round the Sun, not the Sun round the Earth As the Earth moves round the Sun the Sun appears to move against the background of stars Summer and winter constellationsAs the Earth moves around the Sun different constellations are visible at night. The appearance of the sky just after sunset is thus different at different times of the year. The eclipticAs the Earth moves around the Sun it traces a path through the sky. This path is called the ecliptic. Note that is has nothing to do with eclipses or ellipses. Note that the constellations that the sun passes in front of are the signs of the Zodiac as used by astrologers. Direction of revolutionThe direction of revolution around the Sun is the same as the direction of rotation around the Earth (ignoring the 23½ degree tilt of the Earth's axis for the time being) If you look down from above the North Pole the revolution and rotation are both counterclockwise. Later in the course we will find that many other bodies in the solar system show the same sense of revolution/rotation. The rotation is a clue as to how the Solar System was formed.
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2.5 The Origin of SeasonsA number of important effects are caused by the fact that the earth's rotation axis is tilted with respect to its orbit round the Sun Tilt of rotation axisThe tilt is 23½ degrees. The value of this tilt was set at the time of the formation of the Earth The Earth's axis points in the same direction throughout the year (to Polaris) There is a slow change in the direction over thousands of years, called precession, but we will not be dealing with that in this course. This is discussed in your book. The Sun's position on the Sky changes from north to south of the Celestial Equator and back as we go through the year. Because of the tilt, the Ecliptic is different from Celestial Equator.
If you are at the Equator the Sun will be to the north of you between March and September, and to the south of you from September to March. At intermediate latitudes (say 45 degrees north) the sky moves in different daily paths across the sky at different seasons Some of the consequences are
Special areas:Northern Hemisphere:
Generally the farther from the equator you go, the bigger the difference between summer and winter. This is why we get only a small seasonal difference in Hawaii The seasons are reversed in the southern hemisphere. In Australia it snows in August, and the midnight Sun is seen around the December months. The ecliptic, which we met before, goes north and south of the celestial equator. Elliptical orbit of EarthThe elliptical orbit of the Earth causes it to be a bit nearer to the Sun in December than in June. The effect of this on our climate is much less than that caused by the tilt of the Earth's axis. Clicker question sunset |
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2.6 Phases of the MoonThe object of this section is to understand why the moon looks so different at different times of the month, and why is is seen at different times of the day at different times of the month. Orbit of Moon round EarthThe moon takes about 28 days to orbit the earth. The orbit is in the same sense as the rotation of the earth and revolution of the Sun, namely counterclockwise when viewed from above the Earth's North Pole. It is tilted by a few degrees from the plane of the Earth's orbit around the Sun. Illumination of MoonThe Moon does not produce light. The changing appearance of the moon is caused by the changing angles of illumination from the Sun. When the illumination is mainly from the back we see a crescent moon. When it is mainly from the front we see a gibbous moon. Note that the illumination (or the phase) of the moon does not change much during the course of a night. New, quarter and full moonsAt new moon the Moon passes between the Sun and the Earth. Because of the tilt of the Moon's orbit with respect to the Earth's orbit around the Sun they are usually not perfectly aligned. Eclipses occur only rarely. At true new moon only the back of the moon is illuminated, so we do not see it. Conventionally we often refer to the moon as new when the first crescent is visible a few days after true new moon. The first sighting of the crescent moon after the true "new" moon is of great significance in the Islamic religion. At this time we can sometimes see the dark part of the Moon faintly illuminated by light reflected off the earth. As we move through the month the moon waxes (its illuminated fraction increases) through first quarter (after one week) to full moon (after two weeks). It is first crescent, then gibbous. From Full to New again the moon wanes through gibbous as far as third (or last quarter) and crescent to new moon again. When can you see the Moon?Assuming that you are in an "ordinary" part of the Earth (not near the poles)
First quarter best for moon viewing.
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Locked rotation of MoonMoon keeps same face always pointing towards earth.. This is why we can (in principle!) recognize the "Man in the Moon" The only people who have seen the back of the Moon are astronauts. We will discuss the reasons for the locked rotation later on in the course. |
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2.7 EclipsesEclipses occur when the Earth and Moon are well-enough aligned to form shadows on each other Because the Moon's orbit around the Earth is tilted a few degrees from the Earth's orbit around the Sun, we don't get eclipses every month. Solar EclipsesSolar Eclipse occurs when the moon comes in front of the Sun The angular diameter of the Moon is almost the same as the angular diameter of the Sun, so you have to be almost perfectly lined up to see a total eclipse. A total eclipse only last a few minutes, max. The effect is spectacular. Solar eclipse occurs only at New Moon. The angular diameter of the Moon (and to a lesser degree, the Sun) vary because the orbits of the Moon and Earth are ellipses. The angular size of the Moon therefore changes by about 10%. Sometimes the angular diameter of the Moon is larger than the Sun: then you get a total eclipse; if it is smaller than the Sun you get an annular eclipse, which is less spectacular. Partial eclipse is visible over a large area of the Earth. Total eclipse is visible only in a small area. you have to be within a few miles of a line to see a total eclipse, but you can see a partial eclipse over a much wider track. Web page with dates of future eclipses
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Lunar EclipsesLunar Eclipse occurs when the Earth comes between Sun and Moon. It can occur only at Full Moon During the eclipse, the Moon looks much darker than usual, but if you aren't looking at the moon you could miss the eclipse. A Lunar Eclipse is visible by anyone on Earth who can see the Moon, ie half the world The Moon is still faintly visible during a total lunar eclipse, due to scattering of sunlight round the edge of the Earth through its atmosphere. The moon then glows with a faint red color.
Clicker question Lunar Eclipse Clicker question: Solar Eclipse
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