2. Modern: A constellation is a region in the sky. Every object in the sky, whether we can see it or not, is part of a constellation.

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1 6/ Star Cluster size about to m importance: where stars are born composed of stars. 11. Galaxy size about m importance: provide a stable environment for stars. Composed of stars. 12. Galaxy Cluster size about m importance: provide a stable environment of galaxies. Composed galaxies. 13. Superclusters of Galaxies size about m importance: provide a stable structure for galaxy clusters. Composed of galaxy clusters. 14. The Universe size big size of visible universe about m importance: it contains everything there is. Composed of everything there is. How Science is Done The Scientific Method The scientific method, per se, is not followed but its spirit is. 1. Observation Newton sees apple fall from tree. 2. Hypothesis a force called gravity applied by Earth on the apple caused it to fall. 3. Prediction This gravity is the same force that keeps the Moon in orbit about the Earth. 4. Experiment Measure the motion of the Moon. 5. Conclusion If the results of the experiment disagree with those of the prediction that is, the prediction lies outside the uncertainty in the measured number, the hypothesis must be rejected. If the results agree within experimental uncertainty, the hypothesis is supported but not proved. A hypothesis can never be proved correct because of experimental uncertainty. Chapter 1 - Discovering the Night Sky Constellations: 1. In ancient times, a constellation was a pattern of stars in the sky. 2. Modern: A constellation is a region in the sky. Every object in the sky, whether we can see it or not, is part of a constellation. 3. There 88 constellations that cover the entire sky both northern and southern hemispheres. 4. Since a constellation is no longer a pattern of stars, we need a new name. A pattern of stars is an asterism. Bayer System for Naming Stars based on constellations. Uses the possessive form of a constellation name along with a letter from the Greek alphabet in order from brightest (alpha) to dimmest (omega).

2 The brightest star in the constellation Orion is Alpha Orionis. Alpha Centauri is the brightest star in the constellation Centaurus. Celestial Sphere Ancient model of the universe still useful today. Ancient Observations: 1. They could not detect the motion of the Earth. They could not detect the parallax of the stars. Parallax is the apparent shift in position of an object relative to a more distant object due to a change in perspective of the observer. But the ancients didn t know how far away the stars are and how small their parallaxes are. In addition, they could not feel the motion of the Earth much like a passenger in a car with eyes closed can tell when it turns. They concluded that the Earth was at rest it was not moving. 2. The Greeks took the Earth to be at rest at the center of the universe. 3. The Greeks knew that the Earth was a sphere. They saw that during the eclipse of the Moon, the Earth always cast a circular shadow only possible if the Earth were a sphere. In addition, ships coming into to port would first show the top of the mast with the rest of the ship coming into view as it closer and closer. 4. When looking at the night sky, it appeared to the Greeks as if they were looking at the inside of an even larger sphere. They called this the Celestial Sphere. 5. They imagined the stars attached to the inside of this sphere. The celestial sphere would rotate once on its axis, carrying the stars with it. It rotated from east toward west so that stars rise in the east and set in the west. According to the ancients, the stars were all the same distance away. We know that they are at a variety of distances, but that is irrelevant as far as deciding which direction to point a telescope. We still us the Celestial Sphere today. Some of the locations on the CS (celestial sphere) are reflections of the same locations on the Earth. The Earth has two poles: the north geographic pole (NGP) and the south geographic pole (SGP). Directly above these poles on the CS are the north celestial pole (NCP) and south celestial pole (SCP). Directly above the geographic equator GE on the CS lies the celestial equator (CE). The Sun follows a path on the CS called the ecliptic, completing a trip around the sphere in one year. It is a circle on the CS and makes an angle of 23.5 degrees with respect to the CE. (Picture Below) Locating Places on the CS

3 Use a system that is similar to that used on Earth latitude and longitude. We measure latitude north and south from the equator ( 0 degrees latitude) from south 90 degrees to north 90 degrees. Imagine cutting the Earth like a tomato. We measure longitude east and west around the equator from a standard location on the Earth. Lines of longitude are all great circles that intersect at the poles. Cut the Earth like sections of an orange. We measure from the prime meridian the line of longitude that passes through the former observatory at Greenwich, England. We measure east from 0 to 180 degrees and west from 0 to 180 degrees. On the CS Declination like latitude: d or it is measured from 0 to 90 degrees north and south from the CE, with plus being north and minus being south. Right Ascension like longitude: RA it is measured around the CE toward the east from a celestial prime meridian from 0 to 24 hours. We use time units because it gives information about the rising time of objects we might wish to observe. On the CS, the prime meridian is the one that passes through the vernal equinox. Here, the vernal equinox is the location of the Sun when it is highest in the sky on the first day of spring. Easy to find the vernal equinox in the northern hemisphere it runs right along the edge of Cassiopeia just of the edge of Caph. Local Coordinates: altitude and azimuth. Altitude number of degrees above the horizon. Azimuth number of degrees around from the east one should rotate to point toward an object. Problem: these local coordinates are different for different places on Earth and they change with time. Declination and Right Ascension change only very slowly with time. Due to the precession of the Earth s rotation axis takes 26,000 years for one complete precession. We recalculate d and RA every 50 years.

4 Astronomy and Time Keeping Earthly Cycles The Day There are two days!! the sidereal day and the solar day. Sidereal day time for the Earth to complete one rotation on its axis relative to the stars. Solar day time for the Earth to complete one rotation on its axis relative to the Sun Sun goes from highest in the sky back to highest in the sky. Because the Earth revolves around the Sun, these two days are slightly different. Note on terminology: Rotation always refers to a body spinning on its axis; revolution or revolving always refer to one body in orbit about another. How much longer is the solar day than the sidereal day? Since there are days in a year and 360 degrees in a circle, the Earth moves about 1 degree per day along its orbit. Let this one degree movement be a sidereal day. To get to solar day, the Earth must rotate an additional degree on its axis. As we have shown 1 degree is 4 minutes. The solar day is about 4 minutes longer than a sidereal day actual number is 3 minutes 56 seconds. Note that, since solar time is the civil time that we keep, stars rise and set 4 minutes earlier each day. About a half hour a week. About 2 hours each month. This is about 24 hours in a year cycle repeats. Time Keeping Around the World Until the about the middle of the 19 th century, every place kept its own local time. Problem with using solar time is that it is not constant the Earth speeds up when closer to the Sun and slows when farther from the Sun and this causes the length of the solar day to vary. We use mean solar time take the length of all the solar days during a year and use the average for our solar day. With the coming of the railroads, a different system was required to reduce complexity zone time. Divide the Earth into 24 time zones along lines of longitude 15 degrees apart. The civil time throughout the zone is taken to be the mean solar time at the center of the zone. When crossing a zone boundary heading east, we add an hour.

5 When crossing a zone boundary heading west, we subtract an hour. International Date Line takes care of problems of adding or subtracting hours while traveling and getting back to one s point of origin on the wrong day. Cross the International Date Line toward the east add an hour and subtract a day. Cross the International Date Line toward the west, subtract an hour and add a day. The Year time of the Earth to complete one orbit about the Sun slightly less than days. According to Allen s Astrophysical Quantities: days from equinox to equinox. Month based on the orbit of the Moon the Moon takes about 30 days to complete one orbit about the Earth. Seasons The plane of the Earth s orbit about the Sun is called the ecliptic plane. The Earth s axis of rotation is not perpendicular to the ecliptic plane the angle between the Earth s rotation axis and the direction perpendicular to the ecliptic plane is 23.5 degrees the same as the angle between the celestial equator and ecliptic on the CS. It is this tilt of the Earth s axis that causes the seasons. It is summer in the northern hemisphere when the north pole leans toward the Sun and winter when it leans away from the Sun. Summer Solstice first day of summer on or about June 21. Autumnal Equinox first day of autumn on or about Sept. 21. Winter Solstice first day of winter on or about Dec 21 Vernal Equinox first day of spring on or about Mar 21. Note that it is hotter in summer because the Sun is higher in the sky and more effective in transmitting energy to the Earth and for a longer time. In the winter it is lower in the sky for a shorter period of time. In fact, in the northern hemisphere, the Earth is farther from the Sun in the summer than it is in the winter. Calendar

6 It is important to keep the calendar synchronized with the seasons difficult because of that extra quarter day in the year. The Romans developed the Julian calendar to take care of the extra day by adding an extra day to February every four years. But its not a quarter of a day; it s slightly less than a quarter of day. By the 16 th century, the calendar was notably out of synch with the seasons. By 11 days. Pope Gregory did two things 1. He chopped 11 days out of the month of October. 2. He put together a group of astronomers and mathematicians to develop a new calendar the Gregorian Calendar. The Rules for the Gregorian Calendar: 1. Every year has 365 days unless it is evenly divisible by 4 in which case it has 366 days 2. Unless it is evenly divisible by 100, in which case it has 365 days 3. Unless it is also evenly divisible by 400 in which case it has 366 days. This calendar will be off by a day after 3300 years. The Phases of the Moon A sidereal period time for the Moon to complete one orbit about the Earth relative to the stars days. Synodic period time for the Moon go through a complete cycle of phases days. The synodic period is longer than the sidereal period for the same reason the solar day is longer than the sidereal day. To be included on a diagram of the lunar phases: 1. Orbit of the Moon with the Earth at the center. 2. Eight different locations of the Moon in its orbit about the Earth. 3. For each location, I will show what the Moon looks like to an observer on Earth. 4. Name of the phase. 5. Relative time the phase occurs relative to the New

7 Moon. 6. Rough estimates of the rising and setting times of each phase. Notes: 1. Strictly speaking, the new, quarter, and full phases only exist for an instant. We will go by eye; if it looks full, we ll call it full. 2. Sizes of crescent and gibbous phases change over time. 3. Hard to see small crescents when the Sun is in the sky. 4. How to tell if a gibbous or crescent phase is waxing or waning: a. Both the Sun and the Moon appear to drift from west toward east against the background of the fixed stars on the celestial sphere (CS). b. Note that it takes the Sun 1 year to make one complete circuit around the ecliptic while it only takes the Moon 1 month to go around its path on the CS. c. This means that the Moon moves 12 times faster than the Sun. d. If the Moon in the sky is to the west of the Sun, the Moon moves toward the Sun and whatever phase it is in, it will be a waning phase. e. If the Moon in the sky is to the east of the Sun, the Moon moves away from the Sun and whatever phase it is in, it will be a waxing phase.