PHYS 1051 Intro Astronomy Astronomy Today, 9th Ed. Chapter 1 Discussion 2-9 Due on week 2 [1. What does an astronomer mean by "the universe"? The universe can be defined as everything: the sum total of all space, time, and matter that exists. However, most astronomers will use the word "universe" to refer to everything which can be observed. Two definitions are needed because we may be a part of a "multi-verse" containing many universes that are beyond the reach of our observations. Our universe has a finite age (13.7 Gyr), and light can only travel so far in that time, thus we can only see to a finite distance. Later, we will discuss "inflation" theory which suggests that even our own bubble universe is mostly beyond the reach of observations.] -------------------------------------------- 2. Compare the size of the Earth with that of the Sun, Milky Way Galaxy and the entire Universe. D_E X 110 = D_Sun D_Sun X 10^11 = D_M.W. D_M.W. X 10^5 = D_Univese 3. What is the scientific method, and how does science differ from religion? The scientific method is a process for discovering the best possible explanation for natural phenomena. The process begins when obs- ervations lead to the formulation of a hypothesis, a preliminary explanation that makes testable predictions. Experiments are run to test the hypothesis. If they do not agree, the hypothesis is discarded or modified. The new hypothesis is tested, and so on. Gradually a coherent theory is formed to explain and predict a variety of observable phenomena. The rejection of old ideas based on new data is the main difference between scientific belief systems and religous ones. [ 4. What is a constellation? Why are constellations useful for mapping the sky? A constellation is a designated region of the sky which contains one or more asterisms. These are a useful way to divide up the sky into roughly square subsections. Constellations do not only include the stars connected by lines on star maps. Every point on the sky, star or not, falls in one of the 88 constellations defined internationally. Constellations are named after some asterism contained within its line-segment boarders (e.g., Ursa Major contains a Great Bear). This helps make constellations easier to identify. Constellations also provides a means of naming the star, e.g., the brightest star in Orion will be "alpha Orionis", the second brightest is "beta Orionis", etc. ] 5. Describe the daily and annual motion of the Sun, Moon and stars as viewed from the Earth. On a daily basis, the Sun, Moon and stars rise roughly East and set roughly West. This appearance results from the Earth's spin because the Earth's surface is rotating into the East. The Moon and Sun have gradual motions relative to the stars, with the Moon moving about 12 degrees per day and the Sun moving 1 degree per day. The Moon's motion is due to it's orbit around Earth. The Sun's motion relative to the stars is due to the Earth's orbital motion. The motions of the Sun and the Moon relative to the stars take them above and below the celestial equator which leads to changes in their rising and setting times, and their time spent above the horizon. So on a yearly basis, the Sun get's higher above the horizon in summer and lower in the winter. 6. How and why does a day measured with respect to the Sun differ from a day with respect to the stars? The rotation period with respect to the Sun is the solar day, while it is the sidereal day when measured with respect to the stars. The solar day is 3m56s longer than the sidereal day because the Earth is revolving around the Sun CCW as it rotates CCW. That means it must rotate farther (by 1 degree) to line up with the Sun than the stars. 7. Why do we see different stars at different times of the year? It helps to choose a time to explain this, so consider looking at the sky at midnight. If you look at the intersection of the ecliptic with the meridian (that is above your southern horizon), then you are looking roughly along a line from the Sun that goes through the Earth and points to the stars in front of you. As the Earth revolves around the Sun, that line points to different constellations. Thus, you see different stars at midnight on different dates. Similarly, the Sun is superimposed upon different zodiacal constellations throughout the year (you just can't see them behind the Sun because the Sun lights up our sky.) 8.What is precession and what causes it? Precession is a wobbling of the Earth's spin axis whereby it points towards different stars along a circular path and makes a complete cycle in 26,000 years. Precession is also the westward motion of the vernal equinox along the celestial equator of 50" per year. Precession is caused by the gravitational pull on the Earth's equatorial bulge by the Sun and (especially) the Moon. If the Earth were not tilted relative to the orbital planes of the Sun and Moon, their would be no precession. 9. Why are there seasons on Earth? There are seasons on Earth because the rotation axis of Earth is tilted 23.5 degrees with respect to the normal to the ecliptic plane. This tilt means that a given location on Earth receives different angles and intensities of sunlight over the course of Earth's orbit. When it is summer at your location, the Sun is almost directly overhead at noon. Therefore, the rays of the Sun's light strike the ground at nearly a 90 degree angle and is very concentrated. In winter, however, the Sun never gets very high, even at noon, and the sunlight is more diffuse on the ground. Besides the changing directness of the Sun's rays, there is also the changing length of daylight hours. During the summer, the Sun is above the horizon over 12 hours, while during winter, the Sun is up less than 12 hours. ------------------------------------------- 10. How much of the Moon's surface is lit by the Sun at any one time? Why do we see different phases of the Moon? One-half of the Moon's surface is lit by the Sun at all times except during a lunar eclipse when the Earth blocks the Sun's light. We see phases because, as the Moon orbits the Earth, the lit portion moves into, and out of view. The lit hemisphere faces the Earth during a full Moon, but it is facing away from Earth during a new Moon. 11. What causes a lunar eclipse? A solar eclipse? Why aren't there lunar and solar eclipses every month? A lunar eclipse is caused by the shadow of the Earth falling on the Moon, while solar eclipses are caused by the shadow of the Moon falling on the Earth. The Moon's orbital plane is tilted by about 5 degrees relative to the Earth's orbital plane around the Sun. Consequently, some months the Moon appears to pass up to 5 degrees above or below the Sun instead of covering up the Sun. Similarly, the Moon can pass above or below the shadow of the Earth, thereby avoiding a lunar eclipse. 12. Do you think an observer on another planet might see an eclipse? Why or why not? Yes they could because other planets have moons which can cast shadows on the planets surface, or move into the shadow of that planet. We have images of the moon Phobos eclipsing the Sun as seen from Mars (by a rover). 13. What is parallax? Give an everyday example. Parallax is the apparent motion or shifting of an object cause by the motion or shifting of the observer. Driving by trees in front mountains, the trees will appear to shift relative to the mountains. Staring at your thumb with your right eye and then shifting to the left eye, the thumb will "jump" to the right relative to the background. 14. Why is it necessary to have a long baseline when using triangulation to measure the distances to objects in space? Because the larger the baseline, the larger the angle that is subtended by the object as it appears to move back and forth. The atmosphere prevents us from resolving angles smaller than about 1", so the larger the parallax angle, the more clearly we will be able to measure the distance to an object. 15. To determine the diameter of a far away object, you need to known the distance to the object and the angle subtended by the object. The diameter is then given by diam = dist X angle where the angle is measured in radians (1 radian = 57 degrees.) -------- End of Week 1 assignment ---------------------------------