Prof. Dale E. Gary
Basic Patterns and Motions of the Sky
Looking at the Sky
You can enjoy astronomy all on your own, just by going out and looking at the sky (day or night!). You do not need a telescope, or charts--all you need is to be armed with a little information. Please go out on clear evenings and look up at the sky. Find the moon if it is up. Unfortunately, all of the planets are near the Sun. If you get up before the Sun (around 5 am) you can see Venus and Saturn, which are bright, low in the Eastern sky.Our View from Earth's Surface
Imagine the Universe as extending out into space in all directions. When we stand on the surface of the Earth, we can only see at most half of the Universe, the other half being blocked by the body of the Earth. We call the part we can see, the sky. Think of the sky as painted, or projected, onto the inside of a dome--like a planetarium dome.Patterns and Sights in the Sky
The directions around the horizon are the familiar directions North, East, South, and West (N, E, S, W). We can specify coordinates, in degrees, by assigning N as 0, and increasing eastward (E = 90), through S (180), W (270) and finally back to N (360). We call such coordinates the azimuth coordinate.
The angle of a star or other object from the horizon is called the altitude coordinate. A star on the horizon has an altitude of 0 degrees. A star straight overhead has the maximum altitude of 90 degrees. Why is this the maximum? Because if a star in the north has an angle of more than 90 degrees, then it has an angle of less than 90 degrees from the south horizon.
We can pinpoint a star or other object using the pair of coordinates, altitude and azimuth, i.e. (30 degrees, 45 degrees) means the star is thirty degrees above the NE horizon. but if we want to be really accurate, we may want to specify the position to better than a degree. We divide degrees into arcminutes (or minutes of arc), which is 1/60th of a degree, and arcseconds (or seconds of arc), which is 1/60th of an arcminute, or 1/3600th of a degree. For shorthand, we use the symbols ' and ". Thus, an altitude of 35d 27' 15" means an angle of 35 degrees, 27 arcminutes, and 15 arcseconds. So remember, 1 degree = 60' = 3600".
Meridian Line and Zenith
The point directly overhead is called the zenith. Note that if the Sun is ever at the zenith, then your shadow would be directly under you. This never happens in Newark, but in some places on the Earth it does, which we will discuss later. The imaginary line from the north point on the horizon through the zenith to the south point on the horizon, is called the meridian or local meridian. This is a very important line in the sky, as we will learn shortly.
When you see the night sky on a dark night away from city lights, you can see at most about 1000 stars. You may pick out patterns in the stars--in general this is a natural function of the human brain, to find patterns. However, these patterns are for the most part random chance groupings of stars, and the stars are not actually related. Two stars may appear close to each other in the sky, but actually be thousands of light years apart in distance.Earth-center Based Coordinates
Asterisms and Constellations
Ancient civilizations and cultures noticed apparent groupings of stars and gave them names. Usually the names are associated with myths or legends of human or supernatural events. We call such groupings by two names--constellation (a group of stars officially recognized by astronomers) and asterism (a group of stars that form a recognizable pattern, but is not officially recognized). A well-known example is the Big Dipper, which is an asterism and is part of the constellation of Ursa Major (the Big Bear).
Official Astronomical Boundaries
The constellation names were standardized in 1928, to agree with the names known in Europe at the time, but almost all cultures had their own names and stories.
Modern usage of constellations now refers not to a set of stars, but to areas of the sky, within official boundaries set by the committee in 1928. In this way, a faint galaxy found in some area of the sky can be said to belong to the constellation within whose boundaries it lies. For example, the Andromeda Galaxy is found in the constellation of Andromeda.
When you look at a really dark sky, like you might see in the country far from city lights, you can see a faint path of milky light running across the sky. This is the combined light of the billions of faint stars making up our galaxy, and is called the Milky Way. In addition to the light, you may see dark patches where there appear to be fewer stars--these are dust clouds, which are a common feature of spiral galaxies.
Sun, Moon, and Planets
In addition to the far away stars, and the even farther away faint Milky Way, you can also see a variety of nearby objects--members of our solar system. This includes, of course, the Sun itself, as well as the Moon and 5 of the planets, Mercury, Venus, Mars, Jupiter, and Saturn. These 7 objects were called for the latin planetes, meaning wanderers. Did you know that the Sun and Moon were called planets? Nowadays, we would classify the Sun as a star, and the word moon is reserved for members of the natural satellites orbiting planets, but that was not the original meaning. Both the Sun and Moon appear as large objects in the sky, but the other planets appear only as points of light, indistinguishable from stars except by their brightness and their motion (hence wanderers).
Meteors, Comets and Aurorae
You may occasionally see comets if you know where and when to look. Some of you may have seen comet Hale-Bopp, in 1996. On any given night you may also see a meteor, also called a shooting star, or falling star. Most are really just a sand-grain-sized pebble of space dust, actually a piece of a comet, hitting the atmosphere. Several times per year there are meteor showers, which occur when the Earth passes through a comet's orbit. Finally, solar storms can cause aurorae, also called northern lights, which you may see as shimmering rays or curtains of light all over the sky. They are rare in New Jersey, but common in Canada and Alaska.
The altitude and azimuth coordinates are useful for discussing locations of objects with your local neighbors--for example you can call your friend and say, "Look in the west right now, 20 degrees above the horizon, and you will see Venus." But if your friend lives in California, he may well say, "What are you talking about--it is still daylight outside!" So astronomers need a single fixed coordinate system to locate objects so that they can tell other astronomers around the world where the object is. To do this, we use a coordinate system based on the center of the Earth, which we call the Celestial Sphere. Such coordinates are called celestial coordinates. To help keep them straight, use the following table:
Coordinate Name Type Conceptual
Reference Point celestial coordinates absolute celestial sphere center of the Earth altitude and azimuth
local sky dome our local surface of the Earth
The celestial coordinates are based on extending the familiar points on Earth up into the sky, i.e. extend the equator to become the Celestial Equator, extend the north pole to become the North Celestial Pole (NCP), and extend the south pole to become the South Celestial Pole (SCP). We also extend lines of longitude and latitude, but because the Earth spins we have to pick a particular date and time to do the extension. We pick midnight on the first day of spring as the moment when the celestial and Earth coordinates line up.Earth in Motion
One difference in these two coordinate systems is that the celestial coordinates are tilted relative to the local coordinates, by an amount that depends on where we are on Earth. If we are at the Earth's equator, then the celestial equator will go overhead, directly from east through the zenith to the west, the NCP will be on the north horizon, and the SCP will be on the south horizon. If we are at the Earth's north pole, the celestial equator will run all the way around the horizon, the NCP will be at the zenith, and the SCP will be directly under our feet. If we are at some other north latitude, say in Newark, the the NCP will be at some angle from the north horizon, the celestial equator will be on a tilted path from east to west, but not reaching overhead, and the SCP will be below the horizon at the angle exactly opposite the NCP. Note that the altitude of the NCP is exactly equal to your latitude on Earth.
The Ecliptic, path of Sun and planets
The Sun and planets (and the Moon) all follow a path in the sky that is tilted from the celestial equator. This path is called the ecliptic (because it is on this path that eclipses occur). There are 12 constellations along the ecliptic, and these make up the zodiac. The Sun appears in each of these constellations in turn, one per month, and their names may be familiar to you as your "sign" -- the constellation that the Sun is supposed to be in during the month of your birth. This "motion" of the Sun is an apparent motion, caused by the orbit of the Earth around the Sun. The planets follow this same path, because the planets all orbit the Sun in more-or-less the same plane. During the orbits of the planets, the Earth "catches up or falls behind" another planet, so that the path of the planet we see may describe a loop. Normally, the Sun and planets all move eastward in the sky with respect to the stars. When a planet appears to move westward, the motion is called retrograde motion.
Phases of the Moon
The Moon also follows the ecliptic, because its orbit around the Earth is also near the same plane as the planets. As it orbits the Earth, we see it change phase, from New to Full and back again. You should become familiar with the names of the phases (see Phases of the Moon web page). Here is the image from the text: <Moon Phases>.
The apparent motions of the sky are a consequence of the many motions of the Earth, from which we make our observations of the Universe. You may feel that you are quite motionless as you sit in your chair in this room, but nothing could be further from the truth. Here are the motions of the Earth, Sun, and Milky Way Galaxy that are carrying us along at this moment:The Moon
The other planets, moons, comets, asteroids, are also going along their own paths of motion, spinning at their own rate, and so on. It can all be very confusing, but the key to understanding the motions within the solar system is to visualize the motions in a Sun-centered frame.
- Rotation (spin; leads to 24-hour day) -- about 1000 km/h (600 mph)
- Revolution (orbit around the Sun; leads to 365 day year) -- about 100,000 km/h (60,000 mph!)
- Sun's motion through the galaxy (relative to other nearby stars) -- about 70,000 km/h (40,000 mph)
- Sun's orbit around the galaxy (along with other nearby stars) -- about 1 million km/h (600,000 mph!!)
- Galaxy's motion within Local Group (moving toward Andromeda galaxy) -- about 300,000 km/h (180,000 mph)
Sidereal vs Solar Day
The rotation of the Earth causes the Sun, Moon, stars, planets, to appear to rise and set in the sky in a period of roughly 24 hours. The period for the stars to appear to go around once is actually 23 hours, 56 minutes, and about 4 seconds. This period is called the sidereal period, or sidereal day. Note the last four letters form the word REAL, which is a mnemonic to remember that the sidereal period is the REAL period of rotation of the Earth. Because of the orbit of Earth around the Sun, 360 degrees in 365 days, the Sun appears to move about 1 degree each day, so the Earth has to spin one more degree (or about 4 minutes) to line up with the Sun. Adding this 4 minutes to the sidereal day, this makes a solar day of 24 hours, exactly. Actually, however, the Earth's orbit is not a perfect circle, so the time between meridian crossings, which by definition is local noon, varies slightly throughout the year.
Lecture Question #1
Reasons for the Seasons
There is a common misconception about the reason we have seasons (cold in winter, hot in summer). If asked, many people will say that the Earth is closer to the Sun in summer, and farther away in winter. This is not true! In fact, for northern summer, the Earth is at its farthest point from the Sun (in early July).
Some facts about the Seasons for the northern hemisphere:
The seasons are due to the tilt of the Earth's axis relative to the plane of its orbit, by 23.5 degrees. This simple fact is the reason for all of the following:
- First day of spring (vernal equinox) is around Mar 21.
- First day of summer (summer solstice) is around June 21.
- First day of fall (autumnal equinox) is around Sep 21.
- First day of winter (winter solstice) is around Dec 21.
The Earth spins like a top, and like a top, it also wobbles, but VERY slowly. It takes 26,000 years for one wobble. This slow wobble means that the north star (the star Polaris, which is very close to the current position of the NCP) will not always be the north star! When the pyramids were built (about 2000 BC), the star Thuban was the north star. This wobble also causes precession of the equinoxes.
- The northern hemisphere summer starts in June, while the southern hemisphere summer is 6 months later, in January!
- The Sun is up for a longer time in summer (longer days) and a shorter time in winter (longer nights).
- The Sun is up for 6 months at a time at the Earth's poles, and it is night for 6 months.
- The Sun rises higher in the sky in the summer, and is lower in the sky in winter.
Apparent Planetary Motion
We already showed the retrograde motion of the planets due to the orbiting of the planets. Each little circle can be thought of as a little image of the Earth's orbit. This can be seen even better by watching the path of a comet such as Hale-Bopp. Following the path far into the future, you can see the spiral pattern caused by the Earth orbiting the Sun. You can see that any nearby object will show a little circular motion of its apparent position over a 1 year period due to the motion of the Earth. The nearest stars actually show this motion, but it is so small as to be very hard to measure! This is called stellar parallax.
The Moon orbits the Earth, and travels with the Earth about the Sun. Sometimes the Moon gets between the Earth and the Sun, causing solar eclipses, and sometimes the Moon goes into the Earth's shadow, causing lunar eclipses. We want you to have a good understanding of how, when, and why eclipses occur, so pay special attention to this part of the course and work hard to visualize it!
Solar and Lunar Eclipses
As the Moon orbits the Earth, its orbit is tilted slightly (about 5 degrees) from the plane of the orbits of the planets (the ecliptic plane). It crosses the ecliptic plane twice during its orbit. If this crossing happens at the phase of the New Moon, the Moon will be lined up with the Sun and pass in front of it. This alignment has to be perfect in order for the Moon to completely cover the Sun, which happens only for a small part of the Earth. If it lines up perfectly, it is called a total solar eclipse: then the sky will darken just like nighttime, and the stars will be visible. Total solar eclipses are spectacularly beautiful, as seen in the image below.
The 1991 total solar eclipse, Steve Albers
Note: It is often hard for students to see why this doesn't happen every month, and the problem is made worse by drawings such as the one below. We have to use such a drawing so that you can see the geometry clearly, but this top drawing is NOT a scale drawing!
A more accurate drawing is as shown in the second figure, above. On this correct scale, the Earth is the size of a pinhead, the Moon is the size of a grain of sand, and you can see that getting the shadow of a grain of sand to fall on a pinhead is not easy! So total eclipses are rare for any one place on the Earth. But partial solar eclipses (where the Sun is only partly covered by the Moon) occur about once every 6 months.
When the Moon goes to the other side of the Earth (the Moon is a FULL Moon at this time), it can pass through the Earth's shadow. This is called a lunar eclipse. This is a case of a pinhead (Earth) shadowing a grain of sand (Moon), which is much easier to do, so lunar eclipses are somewhat more common than solar eclipses. The following drawing can help to understand when solar and lunar eclipses occur, and why. It shows the Earth-Moon system at several places around the 1-year-long orbit of Earth around the Sun. On each lunar orbit, the Moon is drawn at two positions, new-moon and full-moon. The Moon crosses the ecliptic twice each orbit, along the line of nodes. For half of the orbit, the Moon is above the ecliptic, and for the other half the Moon is below the ecliptic. When the line of nodes is aligned with the Sun, that is when eclipses occur--a solar eclipse at the time of new moon, and a lunar eclipse at the time of full moon.
Here is the image from the text:. <Moon Orbit>.
Lecture Question #2