Exploring the Universe: Basic Concepts of Astronomy

1. Astronomy

2. Chapter 1 Basic Concepts of Astronomy The universe is designed in such a perfect way that celestial objects obey a set of operating laws. Astronomers are supposed to explore and define these laws. An astronomer makes observations and performs calculations in order to find answers to such questions as: • What is the universe composed of? • What is the relationship between the billions of objects in the universe? • What happens inside these objects? • How was the universe created? • What will happen to the universe in the future?

3. 1.1 The Small Revolves Around the Large Each of us know that our small satellite (the Moon) revolves about the Earth. But this is not the only object which revolves in the universe. Most of the planets in our solar system have satellites. These satellites revolve around their planets, while they also rotate aound their own axes. The planets also revolve around the Sun while at the same time rotating about their own axes. This order extends throughout the entire universe.

4. Our Sun is a star which moves within the Milky Way galaxy. A galaxy is a massive group of stars which contains about 100 billion stars on average. Galaxies also form into groups called clusters and clusters are members of larger groups called super-clusters.(cluster: a group of things) The planet and its satellite are the smallest unit in the universe, and from smallest to largest the groups are called; star systems (the solar system for example), galaxies, clusters, and super-clusters. Each member is so small that it is impossible to recognise it in a picture of the group. For example the size of the Sun is so small compared to the size of the Milky Way that we cannot see the Sun individually in a picture of the Milky Way galaxy.

5. 1.2 Observing The Sky With The Unaided Eye Astronomers carry out observations and perform calculations. Observations are carried out either with telescopes or with the unaided eye. Looking at the sky with an unaided eye reveals several objects. Since the sunlight covers the entire sky, it is impossible to make clear observations in the sky during the day-time. Observations must be carried out night time in order to see celestial objects. The most obvious one of these objects is the Moon with its dimensions apparently the same as the Sun. The Moon is the satellite of the Earth.

6. Astronomers carry out observations and perform calculations. Observations are carried out either with telescopes or with the unaided eye. The twinkling, point-like light sources are stars. They appear very small and faint due to the very large distances over which their light travels to reach us. Some stars are even more gigantic than the Sun and there are stars so colossal that they are a million times brighter than the Sun. Each bright point in the photograph is a star or it may be a galaxy.

7. There are also a few other objects that have the appearance of a star, but they do not twinkle. These objects are the planets of our solar system. It is not possible to see all the planets with the unaided eye. A careful observer can make out five planets of our solar system: Mercury, Venus, Mars, Jupiter, and Saturn. Mercury, Venus and Mars can be observed with the unaided eye because of their short distance to us. Jupiter and Saturn can be observed with the unaided eye because of their huge size. A carefull observer can see these planets from the earth with unaided eye.

8. HERE! Morning view from Erbil - 2018 Morning Star: Actually it is not a star. It is a planet. It is the brightest object in sky after the Sun and Moon: Venus. It frequentlyappears in the morning time or evening time. Therefore it is called as the morning or evening star.

9. An observer may see a broad, diffuse band of light through the sky on a clear moonless night. This band is the Milky Way galaxy which consists of about 400 billion stars, and clouds of interstellar gas and dust. Meteor Milky way Galaxy Meteors may be observed with the naked eye. They are seen as brief, falling flashes of light.

10. Comets the other sky objects that revolve around the Sun with constant periods. The most famous comet is the Halley's Comet which passed very close to the Earth in 1986. Since the Earth rotates around its axis, the entire universe appears to be to be in circular motion around the Earth.

11. 1.3 The Celestial Sphere The sky appears to be a sphere. An observer sees the sky as a large sphere which is called celestial sphere. Anobserver cannotdetermine how far the objects are, which one isfurther or, even their positions relative to eachother, all celestial objects appear to be on thecelestial sphere. We cannot say that one star for example is some kilometers away from another. Instead we specify its position by using astronomical coordinate systems.

12. The directions and the separations between the celestial objects are measured in angular units, because there is no easy way to determine the real distances between the objects. The actual positions of the stars are A and B,but they both appear to be on the celestial sphere, at Aand Brespectively.

13. Celestial Coordinates : We describe the celestial sphere using a similar geographical notation: * The North Celestial Pole is the point on the celestial sphere directly above the Earth's North Pole. Similarly, the South Celestial Pole is directly above the Earth's South Pole. * The star Polaris is located very close to the North Celestial Pole. Polaris is therefore also called the North Star.

14. Because Polaris lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole—the north celestial pole—Polaris stands almost motionless in the sky, and all the stars of the northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation and for astronomy.

15. For any position on the surface of the Earth, the point on the celestial sphere that is directly overhead is called the zenith. Zenith point Celestial sphere Shadows of trees when the sun is directly overhead (at the zenith).

16. Quick QUIZ: Show celestial sphere, celestial coordinates, zenith and horizon for the person on the earth.

17. 1.5 Vernal Equinox, Autumnal Equinox. At the times of the vernalequinox (the equinox in spring) and the autumnalequinox, the lengths of day and night at both north and south hemispheres are equal. An equinox is the moment in which the plane of Earth's equator passes through the center of the Sun's disk, which occurs twice each year, around 20 March and 23 September. On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction.

18. 1.6 Summer Solstice, Winter Solstice At summersolstice the North Pole is tilted most towards the Sun, and at wintersolstice the South Pole is tilted most towards the Sun. Either of the two times in the year, the summer solstice and the winter solstice , when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. The vernal equinox occurs on March 21 and the autumnal equinox occurs on September 23.

19. Summer solstice and winter solstice: two times in a year, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. The minimum length of shadow during a day is less in summer than in winter. Summer solstice Noon time shadow Winter solstice Noon time shadow

20. Quick QUIZ: 1. What is solstice? What is equinox? Show the solstices and equinox on a simple diagram below. Quick QUIZ: 2. Draw the shadows of the tree below in winter solstice, summer solstice and equinox at noon time.

21. 1.7 The Moon The angle between the plane of the orbit of the Moon around the Earth and the plane of the orbit of the Sun around the Earth (the ecliptic) is 5°. Since the angle is small, it causes the Moon to stay near the ecliptic as observed from the Earth. The ecliptic is the plane of the virtual orbit of the Sun around the Earth (remember that the Sun appears to revolve around the Earth when it is observed from the Earth). It takes the Moon a little more than 27 days to revolve around the Earth once, in the reference frame of the distant stars (fixed stars). This period is called the sidereal period. (Sidereal: with respect to distant stars) Observing from the Earth, we will measure about 29.5 days for the Moon to complete its full cycle of phases. For example: The time needed from one full Moon phase to the next is about 29.5 days. This period is called the synodic period or the lunar month.

22. The phase of the Moon depends on the relative alignment of the Moon, Earth and Sun. After one complete circle around the Earth (one lunar month) is completed, the Moon requires extra time to reach the initial Moon - Earth - Sun alignment because of the motion of the Earth around the Sun. Synodic or lunar month 29.5 days 2.5 days Sidereal month 27 days (c) Synodic period or lunar month

23. From Earth's surface we can only observe the same side of the Moon; the other side of the moon is not visible from Earth. Since the rotation period of the Moon about its own axis is equal to its period of revolution around the Earth. This type of motion is common in the universe and is called synchronous rotation. Sun

24. 1.7.a The Phases of the Moon

25. 1.7.a The Phases of the Moon What is the difference between moon and star light? One more difference between the Moon and the stars in the sky is that the Moon does not emit its own light, instead it reflects the sunlight. The Moon appear to take on different shapes which are called the phases of the Moon. We have a moonless night when the Moon is between the Sun and the Earth, because the Moon is unable to reflect the sunlight. This phase of the Moon is called the new moon. The phase, in which the full shape of the Moon is observed is called the full moon.

26. The position of the moon relative to Earth and the Sun is called its configuration. When the Moon is between the Earth and the Sun, the configuration is called the conjunction. When the moon is opposite the Sun it is said to be in opposition. The configuration in which the Moon is in a position 90° from the Earth-Sun line is said to be in quadrature.

27. In one complete cycle there is one opposition, one conjunction and two quadratures. If the observable portion of the Moon is less than half, it is called crescent. If the observable portion of the Moon is more than half, it is called gibbous. If the Moon is observed to be exactly one half of a circle, it will either be the firstquarter or thirdquarter. Starting from the new moon; the phases of the Moon are: new moon waxing crescent first quarter waxing gibbous full moon waning gibbous third quarter waning crescent and finally new moon. Viewing from Earth

28. 1.7.b Solar and Lunar Eclipses A Solar eclipse is the phenomenon where the Sun disappears under the shadow of the Moon, and occurs when the Moon is exactly between the Sun and the Earth. Since the apparent diameter of the Moon is the same as that of the Sun, the Moon prevents the sunlight reaching Earth. The Sun completely disappears at the umbral region. Thus a totalsolareclipse is observed in this region. The penumbral region is the region of partial solar eclipse.

29. A Lunar eclipse is the disappearance of the Moon behind the shadow of the Earth. During a lunar eclipse, Earth comes between the sun and the moon so that all or part of the sun's light is blocked from the moon. The Earth does not let the sunlight illuminate the lunar surface, thus, the Moon doesn`t get enough light to be observable.

30. One may think that as the Moon revolves around Earth each month, solar and lunar eclipses should occur once a month. But this does not occur because the orbit of the Moon is tilted by about 5°. Earth’s orbit: ecliptic Therefore the Moon generally escapes the Earth's shadow at opposition and passes just above or below the Sun at conjunction. But the orbital planes of the Moon and Sun intersect two times in each revolution. That is, the Moon passes through the Earth's orbital plane twice a month If opposition or conjunction occurs at these times, lunar and solar eclipses will occur, respectively.

31. Questions 1. What is an astronomer supposed to do? 2. With a naked eye, which celestial objects can you observe at night time? 3. How can an observer, with a naked eye, differentiate between a star and a planet? 4. Define the celestial sphere? 5. Why do we use hours, minutes and seconds instead of degrees in right ascension? 6. What is the main property of the equinoxes? 7. Define the motions of the Moon and the Sun with respect to the entire sky? 8. What is the difference between the sidereal period and the synodic period of the moon? 9. What are the phases of the Moon? 10. Why don't we see eclipses every month?