Chapter 2 Discovering the Universe for Yourself

What does the universe look like from Earth? With the naked eye, we can see more than 2000 stars as well as the Milky Way. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

Constellations A constellation is a region of the sky. Eighty-eight constellations fill the entire sky. Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

The Zodiac Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

Star Patterns… From the earliest days, our ancestors sought to find rhyme and reason for the stars in the heavens and their perceived motions. Religion and superstition defined how Man looked at the stars, because there was no way to formulate a theory and then test it. The tools didn’t exist.

Constellations make locating stars easy

Use the Big Dipper in the northern sky as a way to find other groups of stars

Use the winter triangle to find southern constellations during winter evenings

Use the Summer Triangle to find southern constellations during summer evenings

Angular distances between stars in the sky are measured in degrees

Angular distances between stars in the sky are measured in degrees

Angular Measurements Full circle = 360º 1º = 60 (arcminutes) 1 = 60 (arcseconds) Use this slide if you want to review the definitions of arc minutes and arc seconds.

The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The 88 official constellations cover the celestial sphere. The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful applet for learning about the sky. When discussing this slide, be sure to explain: North celestial pole South celestial pole Celestial equator Ecliptic It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.

The Celestial Sphere North celestial pole is directly above Earth’s North Pole. South celestial pole is directly above Earth’s South Pole. Celestial equator is a projection of Earth’s equator onto sky.

Elements of the equatorial coordinate system on the celestial sphere Vernal Equinox: The position of the Sun on the first day of spring Right Ascension: How far east of the Vernal Equinox an object is located Celestial Equator: The line separating the celestial sphere into northern and southern halves. Declination: How far above or below the celestial equator an object is located.

Why do stars rise and set? The answer to the question is very simple if we look at the celestial sphere from the “outside.” But of course, we are looking from our location on Earth, which makes the motions of stars look a little more complex… Earth rotates from west to east, so stars appear to circle from east to west.

Motion of Stars at the North Pole

Motion of Stars at the Equator

What about the Sun? It seems to move across the celestial sphere on a path called the ecliptic.

Summer solstice Equinox position Winter solstice

Position of the “noon-time” Sun when photographed every two weeks over a year

The “altitude” of the Sun impacts how much energy is received on Earth resulting in seasons.

The seasons are caused by the tilt of the Earth’s axis of rotation

In other words ... The Earth’s rotation on its axis determines the length of the day The Earth’s orbit around the Sun (revolution) determines the length of the year The Earth’s tilt with respect to the ecliptic causes the seasons.

Why doesn’t distance matter? Variation of Earth–Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt. The two notes should be considered optional. If you cover the first note, you might point out that since Earth is closer to the Sun in S. hemisphere summer and farther in S. hemisphere winter, we might expect that the S. hemisphere would have the more extreme seasons, but it does not because the distance effect is overwhelmed by the geographical effect due to the distribution of oceans.

Precession is a slow, circular motion of the Earth’s axis of rotation

Precession causes the position of the NCP to change over a period of 26,000 years.

The changing phases of the Moon originally inspired the concept of the month

Moon Phase Names New Moon Waxing Crescent First Quarter Waxing Gibbous Full Moon Full Moon Waning Gibbous Third Quarter Waning Crescent New Moon

Although the Moon is always lit from the Sun, we see different amounts of the lit portion from Earth depending on where the Moon is located in its month-long orbit.

How long does it take to go from new moon to new moon? How long does it take the Moon to orbit Earth? 27.3 days 29.5 days lunar month synodic month

Eclipses occur only when the Moon crosses the ecliptic during the new or full phase

Lunar Eclipse © 2014 Pearson Education, Inc. This interactive applet goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses. © 2014 Pearson Education, Inc.

When can eclipses occur? Lunar eclipses can occur only at full moon. Lunar eclipses can be penumbral, partial, or total. Use the interactive figure to show the conditions for the 3 types of lunar eclipse. © 2014 Pearson Education, Inc.

Solar Eclipse © 2014 Pearson Education, Inc. This interactive applet goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses. © 2014 Pearson Education, Inc.

When can eclipses occur? Solar eclipses can occur only at new moon. Solar eclipses can be partial, total, or annular. Use the interactive figure to show the conditions for the 3 types of solar eclipse. © 2014 Pearson Education, Inc.

Why don't we have an eclipse at every new and full moon? The Moon's orbit is tilted 5° to ecliptic plane. So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Use this pond analogy to explain what we mean by nodes and how we get 2 eclipse seasons each year (roughly). Note: You may wish to demonstrate the Moon's orbit and eclipse conditions as follows. Keep a model "Sun" on a table in the center of the lecture area; have your left fist represent the Earth, and hold a ball in the other hand to represent the Moon. Then you can show how the Moon orbits your "fist" at an inclination to the ecliptic plane, explaining the meaning of the nodes. You can also show eclipse seasons by "doing" the Moon's orbit (with fixed nodes) as you walk around your model Sun: the students will see that eclipses are possible only during two periods each year. If you then add in precession of the nodes, students can see why eclipse seasons occur slightly more often than every 6 months. © 2014 Pearson Education, Inc.

Summary: Two conditions must be met to have an eclipse: It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). © 2014 Pearson Education, Inc.

Predicting Eclipses Eclipses recur with the 18-year, 11 1/3-day saros cycle, but type (e.g., partial, total) and location may vary. Point out that even though some ancient civilizations recognized the saros cycle, precise prediction still eluded them. Use the colored bands in the figure to illustrate the saros cycle (e.g., red bands for 2009 and 2027 eclipses are 18 yr, 11 1/3 days apart). © 2014 Pearson Education, Inc.