1. PSCI 1414 General Astronomy
Lecture 5: patterns in the night sky
Alexander C. Spahn

2. Patterns in the night sky
This is an exciting time in the history of astronomy. A new generation of telescopes is scanning the depths of the universe. Increasingly sophisticated space probes are collecting new data about the planets and other objects in our solar system. Rapid advances in computing technology are allowing scientists to analyze the vast amount of new data and to model the processes that occur in planets, stars, galaxies, and the universe. One goal of this book is to help you share in the ongoing adventure of astronomical discovery. One of the best ways to become a part of this adventure is to do what other humans have done for thousands of generations: Go outside, observe the sky around you, and contemplate the awe-inspiring universe of which you are a part. In this chapter, we’ll discuss a few key ideas that will help you understand what you see in the sky.
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3. Patterns in the night sky
Shortly after sunset, as daylight fades to darkness, the sky appears to slowly fill with stars. On clear, moonless nights far from city lights, more than 2000 stars may be visible to your naked eye, along with the whitish band of light that we call the Milky Way. As you look at the stars, your mind may group them into patterns that look like familiar shapes or objects. If you observe the sky night after night or year after year, you will recognize the same patterns of stars.
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4. Constellations
People of nearly every culture gave names to patterns they saw in the sky. We usually refer to such patterns as constellations, but to astronomers the term has a more precise meaning: A constellation is a region of the sky with well-defined borders; the familiar patterns of stars merely help us locate the constellations.
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5. Constellations
The modern sky is divided into 88 constellations. For example, the constellation Orion is shown below.
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6. Constellations
The stars of a constellation only appear to be close to one another. Usually, what we see is only a projection effect. The stars of a constellation may be located at very different distances from us.
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7. Constellations
Constellations are useful for navigation. For example, you can use the Big Dipper in Ursa Major to find the north direction by drawing a straight line through the two stars at the front of the Big Dipper’s bowl. The first star you come to is Polaris, also known as the North Star because it is located almost directly above the Earth’s north pole.
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8. Constellations
During winter in the northern hemisphere, you can see some of the brightest stars in the sky. Many of them are in the vicinity of the winter triangle, which connects Betelgeuse of Orion, Sirius of Canis Major, and Procyon of Canis Minor.
A similar feature, the summer triangle, appears in the northern hemisphere during the summer. The summer triangle connects Vega of Lyra, Deneb of Cygnus, and Altair of Aquila.
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9. Diurnal motion
You will find that the entire pattern of stars shifts its position over time. New constellations will have risen above the eastern horizon and some will have disappeared below the western horizon. This daily motion of the stars is known as diurnal motion and is visible in long exposure photography.
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10. Diurnal motion
To understand diurnal motion, note that at any given moment it is daytime on the half of the Earth illuminated by the Sun and nighttime on the other half. Earth rotates from west to east, making one complete rotation every 24 hours. This is why there is a daily cycle of day and night. Because of this rotation, the stars appear to us to rise in the east and set in the west (as do the Sun and Moon).
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11. Diurnal motion
From night to night, the same constellations rise and set, but a few minutes earlier than on the previous night. After a year has passed, the night sky will have the same appearance as it did when you began. This happens because the Earth orbits the Sun. Over the course of a year, Earth makes one complete orbit and the night side of Earth gradually turns toward different parts of the heavens. Stars will typically rise 4 minutes earlier each night.
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12. Calculation check 2.1
If the constellation of Cygnus rises along the eastern horizon at sunset, at roughly what time will it be highest above the southern horizon? Earth rotates once in 24 hours, so when the stars of Cygnus rise in the east at sunset, they take about 12 hours to go from one side of the sky to the other. As a result, it takes about one half of that time, or 6 hours, for stars of Cygnus to move halfway across the sky. Thus, Cygnus will be highest in the sky around midnight.
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13. The celestial sphere
The illusion that all stars are next to one another in the sky occurs because we lack depth perception when we look into space, a result of the fact that the stars are so far away. The ancient Greeks mistook this illusion for reality, imagining the stars and constellations to lie on a great celestial sphere that surrounds Earth.
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14. The celestial sphere
The celestial sphere is a useful illusion, because it allows us to map the sky as seen from Earth. For reference, we identify four special points and circles on the celestial sphere: North celestial pole – the point directly over Earth’s North Pole. South celestial pole – the point directly over Earth’s South Pole. Celestial equator – a projection of Earth’s equator into space. Ecliptic - the path the Sun follows as it appears to circle around the celestial sphere once each year. It crosses the celestial equator at a 23.5° angle, because that is the tilt of Earth’s axis.
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15. The celestial sphere
The point in the sky directly above an observer anywhere on the Earth is called that observer’s zenith. The zenith and celestial sphere for an observer at 35° north of the equator is show. The zenith appears at the top of the figure, so the Earth appears to be tipped.
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16. The celestial sphere
Stars sufficiently near the north celestial pole revolve around the pole, never rising or setting. Such stars are termed circumpolar stars.
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17. The celestial sphere
Our lack of depth perception on the celestial sphere means we have no way to judge the true sizes or separations of the objects we see in the sky. However, we can describe the angular sizes or separations of objects without knowing how far away they are. An angle measures the opening between two lines that meet at a point. A basic unit to express angles is the degree. A full circle is divided into 360°, and a right angle measures 90°.
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18. The celestial sphere
The angular distance between a pair of objects in the sky is the angle that appears to separate them. You can use your outstretched hand to make rough estimates of angles in the sky.
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19. The celestial sphere
For more precise astronomical measurements, we subdivide each degree into 60 arcminutes and subdivide each arcminute into 60 arcseconds. We abbreviate arcminutes with the symbol ′ and arcseconds with the symbol ″. For example, we read 35°27′15″ as “35 degrees, 27 arcminutes, 15 arcseconds.” 1° = 60 arcmin = 60’ 1’ = 60 arcsec = 60”
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20. The celestial sphere
An object’s angular size depends on its linear (actual) size and distance.
An object’s physical size does not change as you move it farther from your eye, but its angular size gets smaller, making it appear smaller against the background.
An object with an angular size α (in arcsec) is a distance d away. If the angle is small, the linear size D of the object is given by the small-angle formula:
𝐷= 𝛼𝑑 206,265
The number 206,265 is the number of arcsec in a full 360 degrees divided by 2*pi.

21. For next time… Read sections 2.5 – 2.8
Homework: 1. Finish Homework 3 (Due tomorrow, Tuesday)
2. Homework 4 will be posted tonight (Due Friday)
Exam(s): Include planetary studies and have one exam or separate them and have two?