Astronomy

Homework

# Scholars Online Astronomy - Chapter 1 Introduction to Astronomy

## Homework

Reading: Astronomy, Chapter 1: Astronomy and the Universe

• Section 1: Scientific methods guide astronomical observations as investigators move from hypothesis to experiment to theory, but in astronomy, experiment must often be replaced with model development. Using the principles of physics, astronomers simulate the behavior of celestial objects.
• Section 2-4: By studying planets, stars, and galaxies, astronomers attempt to determine the life cycles of solar systems, star systems, and the universe itself.
• Section 5: Astronomers use angular measurements to locate stars relative to specific points in the sky. A full circle has 360 degrees; each degree can be divided into 60 minutes of arc, and each minute of arc can be divided into 60 seconds of arc. Do not confuse minutes and seconds of arc (parts of a degree) with minutes and seconds of Right Ascension (parts of an hour of RA). 1 hour RA = 15 degrees of arc. 1/60 hr RA = 1 min RA = 1/60 of 15 degrees = 15 minutes of arc. What this means in practical terms is that in 1 minute of time, the sky turns 15 minutes of arc. You can test this by looking at the moon -- approximately 0.5 degrees or 30 minutes of arc wide. It should take the full moon 2 minutes to rise completely above the horizon.
• Section 6: Scientific notation (using powers-of-ten) allows astronomers to write large numbers unambiguously. 3.154*102 means a number accurate to four digits: 315.4.
• Section 7: As with most sciences, astronomy has its own units. For distance, these are km, astronomical units (AU), light years (ly), and parsecs (pc).
• Section 8: Required standard pep talk about how great astronomy is. This section is in the tradition of everyone since Plato, who wrote in the Timaeus that the study of the stars lifts men's minds above the mundane.
• Box 1: Small angle formula: D = αd / 206,265
• Box 2: Powers of ten rules: 10a * 10b = 10(a+b) and 10a/10b = 10(a-b)
• Box 3: Converting units: if x unitA = y unitB, then x unitA/y unitB = 1. We can multiply by "1" without changing the value, so we can convert units from one form to another.This first chapter of our text presents an overview of the course, listing the objects we will study and some of the common ways of describing them. As a result, it appears somewhat easier than the following chapters, but don't let appearances fool you. If you have any problems with the concepts presented here, be sure that you ask questions in class and then practice using them until you feel comfortable with them.

### Key Formulae to Know

• Unit conversions:

1 AU = 1.496 * 108km (about 150 million km) = 92.96 * 106 miles (about 93 million miles).

1 LY = 9.46 * 1012km (about 10 trillion km or 1013) km) = 63,240 AU

1 pc = 3.09 * 1013km (about 3 trillion km) = 3.26 ly [or to reverse that, 3 PC is about 10 LY]

• Small Angle Formula:

D = linear distance (length of object perpendicular to line of sight)
d = distance to object along line of sight
α = angular size of object in arc seconds (1/60 of 1 minute of arc)

Note: 206265 arcseconds = 57.296 ° = 1 radian

### Web Lecture

Read the following weblecture before chat: Introduction to Astronomical Concepts and Methods

### Study Activity

Planetarium program exercise

These tasks should be possible on any computer-based planetarium program. You will need to review the user guide for your program to identify exactly how to do each one.

• Check the date and time on the display screen. By default, this should be the same as your system time, but you can control it (see below).
• Use your location setting method to enter your latitude and longitude or find a city near you to use as your home location.
• Use your date and time tools to advance the clock forward and backwards, or to run the display at a rate faster than normal time. When you have finished experimenting, set the time to 9pm for today's date.
• Find and set your field of view size. In most cases, you can set this to 180°, 270°, or 360° around your horizon, and control the range above or below your horizone (-90° to +90°).
• Find and set your horizon to display so that you can see the horizon line and the star field above and below it. Some programs default to a "solid" horizon, so you may need to set this. Turn on direction markings (N, E, S, W) so that you know which direction you are looking.
• Turn on the planetary displays.
For Tuesday night, Sept 11, 2018, here is something close to what you should be seeing:
• Sky Safari at 9pm, for Seattle, WA, with horizon set to transparent and line:

• Starry Night at 8:50, for Seattle, WA, with horizon set to transparent.

Your sky may be different, depending on your location, but just before sunset, you should be able to see Mars just above the southern horizon, with Saturn fainter and to the right of it. Jupiter will be on the western horizon. If you go out about an hour earlier, you may be able to see Venus setting in the west; it should be quite easy to pick out despite the sun's glare.

• Find the tool to enter a specific object's name. Type in "Vega" (the brightest star in Lyra) and have your display center on Vega. See if you can still find the planets in relation to this star and the constellation Lyra.
• Using your program, determine what constellations will be rising (due east), on your meridian above the horizon (due south), and setting (due west) for some night within the next two weeks where you might be able do some observing.

Optional websites:

While astrology is NOT the subject of this course, astrological symbols are often used as a "shorthand" on astronomy charts, so it is work becomeing familiar with them.

Constellation symbols for those in the zodiac:
See the Wikipedia article on the Zodiac, and scroll down to thetable that matches the symbols with the constellation name. Although astronomers are not usually also astrologers, they use these signs as a shorthand on maps and when taking notes.

• Check out the planetary symbols used by both astronomers and astrologers: these can be a useful shorthand when marking observing maps and charts.
• Visit the Powers of 10 site to view the change in scale.
• Visit the online planetarium at Fourmilab and enter your latitude and longitude (if you don't know them, you can use Lat-Long Finder to discover them), then click on "Make Sky Map". The planetarium will show you the sky for the current time.
• Note that this is a map for you to use when facing south and looking up. North is at the top and south is at the bottom but east is on the left and west is on the right. Stars on the right are setting and stars on the left are rising. Planets to the right of the sun are "morning planets": the set before the sun sets and rise before the sun rises. To view them, you have to get up before dawn. Planets to the left of the sun are "evening planets" and will set after the sun sets....at least for a month or two, depending on the motion of the planet and the motion of the sun.
• What constellations are visible?
• Is the moon up? What phase is it in? What part of the sky is it in (east, west, overhead, not visible)
• What planets are above the horizon? (use the symbols chart above to identify the planet!) What part of the sky are they in?
• Notice that the time is showing as Universal time. Change it so that will show you the sky at 9pm at night for the day you do this exercise. For example, if it is now 7pm and the UT is 2:35:32, add 2 hours to the time, then click the "Update" button below the display. What happens to the locations of the moon and planets?
• Select a visible planet and click on it. What constellation is it in?

### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
• Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

### Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

### Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.