Our Place in the Universe “I don’t pretend to understand the Universe. It’s a great deal bigger than I am” - Thomas Carlyle

The view from Earth With the naked eye, we can see more than 2,000 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.

Time Lapse View of MW

Our cosmic address: Here at the beginning of the semester, our main goal is simply to make sure that students understand the basic hierarchy of structure from planet to solar system to galaxy to universe (clustering is less important at this stage). You might also wish to note that this slide shows a lot about human history. E.g., we first learned that Earth is a sphere some 2,500 years ago; we learned that Earth is a planet going around the Sun only about 400 years ago; and we learned that the Millky Way is only one of many galaxies with the work of Hubble some 80 years ago…

Measuring Distance In the solar system distance is measured in astronomical units (AU). 1 (AU) = the average distance between the earth and the sun 1 AU = 150,000,000 km = 93,000,000 mi.

Destination Light travel time The speed of light = 300,000 km/s Destination Light travel time Moon 1 second Sun 8 minutes Sirius 8 years Andromeda Galaxy 2.5 million years Point out how fast the speed of light is: could circle Earth 8 times in one second…. Also note that the speed of light is always the same…

We see objects as they were in the past: The farther away we look in distance, the further back we look in time. At great distances, we see objects as they were when the universe was much younger.

Outside the solar system distance is measured in light years. A light year is the distance light travels in one year. 1 light year = 9,500,000,000,000 km (about 10 trillion km or 6 trillion miles).

The nearest star to our sun is Proxima Centauri, 3.9 light years away To scale it, if both stars are the size of a grapefruit and the sun was on the west coast, Proxima Centauri would be on the east coast.

Even larger distances can be measured in parsecs. 1 parsec = 3.26 light year.

Angular measures are used to describe positions & apparent size Ex: The angular diameter (or angular size) of the Moon is ½°.

Recall that - 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.

Pretend there are lines from your eye to each of two stars, the angle between the lines is the angular distance between the two stars.

The adult human hand held at arm’s length provides a means of estimating angles

Angular Size An object’s angular size appears smaller if it is farther away Use this slide if you want to review the definitions of arc minutes and arc seconds.

Altitude of the celestial pole = your latitude Show students how to locate the NCP and SCP, and how the sky moves around them. (You might wish to repeat the time exposure photo of the sky at this point to re-emphasize what we see.) Can also ask students where they’d find the north celestial pole in their sky tonight…

Prominent constellations can be used to find your way around the sky.

Figure 2-6 The Big Dipper as a Guide The North Star can be seen from anywhere in the northern hemisphere on any night of the year. This star chart shows how the Big Dipper can be used to point out the North Star as well as the brightest stars in two other constellations. The chart shows the sky at around 11 P.M. (daylight savings time) on August 1. Due to the Earth’s orbital motion around the Sun, you will see this same view at 1 A.M. on July 1 and at 9 P.M. on September 1. The angular distance from Polaris to Spica is 102°.

Figure 2-7 The “Winter Triangle” This star chart shows the view toward the southwest on a winter evening in the northern hemisphere (around midnight on January 1, 10 P.M. on February 1, or 8 P.M. on March 1). Three of the brightest stars in the sky make up the “winter triangle,” which is about 26° on a side. In addition to the constellations involved in the triangle, the chart shows the prominent constellations Gemini (the Twins), Auriga (the Charioteer), and Taurus (the Bull).

Figure 2-8 The “Summer Triangle” This star chart shows the eastern sky as it appears in the evening during spring and summer in the northern hemisphere (around 1 A.M. daylight savings time on June 1, around 11 P.M. on July 1, and around 9 P.M. on August 1). The angular distance from Deneb to Altair is about 38°. The constellations Sagitta (the Arrow) and Delphinus (the Dolphin) are much fainter than the three constellations that make up the triangle.