# Astronomy 115: Introduction to Astronomy San Francisco State University

## Homework Solutions #1

• A.) Planet: Body in space which orbits a star (eg the Sun). Traditionally, nine planets orbiting the Sun have been recognized. The official definition of planet was changed recently so that the planet Pluto, and other similar bodies are now called "dwarf planets". Planets orbiting other stars are called "extrasolar planets"
• B.) Star: A large gaseous sphere which produces energy and light by nuclear fusion.
• C.) Solar System: The all the planets collectively, including the Sun, Moon, asteroids and comets. Ie, the Sun and all that orbits it.
• D.) Galaxy: An assemblage of millions to billions of stars together with gas and dust. The Sun, and our solar system reside a galaxy called the Milky Way.
• E.) Universe: Everything; All which is, including all galaxies, and all space between them.

1. The speed of the light is 300,000,000 meters per second (m/s).

A.) Express this number in kilometers per second (km/s).

Unit Conversions: We can always convert from one set of units (eg, km) to another (m) by multiplying by a fraction equal to 1. Since 1000m = 1 km, the fraction:

(1 km / 1000 m) = 1

Hence
300,000,000 m/s *( 1 km/1000 m) = 300,000 km/s

B.) Use a conversion factor (1 km = 0.6 miles) to find how many miles light moves in one second. Similarly, since 1km = 0.6 miles

300,000 km/s * (0.6 miles / km ) = 186,000 miles per second

2. If you lived on Equator (eg. in Quito, Ecuador, Nairobi, Kenya or in Singapore) where would you need to look to see the North Star?

Your Horizon can be viewed by looking straight out all around you. It is perpendicular to your zenith, which is the point directly above one's head. Viewing from the North Pole of Earth, POLARIS (the North Star) is at the zenith.

The Celestial Equator is a line the sky directly above the Earth's equator. If you live on the Earth's Equator, you'll see the celestial equator above your head at the zenith. Perpendicular to this, you'll see the North Star, Polaris on the Horizon. (South of the Equator you can't see the North Star.)

3. What is the ecliptic, and why is it tilted with respect to the equator"
The ecliptic is the plane in which the Earth orbits the Sun. Or, as seen from the Earth, the Ecliptic is a line in the sky in which the Sun always appears. The Celestial Equator is a line in the sky above the Earth's equator. It is perpendicular to the axis of the Earth. But Earth's axis is not perpendicular to the Ecliptic Plane. The Ecliptic and Equator are by tilted about 23.5 degrees with respect to each other.

4. For a solar eclipse to occur, the Moon's shadow must be cast on the Sun. This can only happen at New Moon phase. Those directly in the shadow, on the "Path of totality" see the Sun disappear. The August 21 2017 eclipse was total for those along the path of totality, but parital in San Francisco. It was also very cloudy in SF that day, so most people missed it!
5. Seasons are caused by the tilt of Earth's orbit with respect to the ecliptic plane. As Earth orbits the Sun this tilt causes the Sun to shine more directly on some parts of Earth than others. These parts of the world see the Sun in the sky for more hours of the day, and thus experience hotter temperatures in summer.

6. A.) The Earth orbits the Sun once each year.
B.) The Earth rotates on its axis once each day.
C.) The Moon orbits the Earth once each month.

7. How many seconds are there in 1 year?

(1 year) x (365 days/year) x (24 hours/day) x (60 minutes/hour) x (60 seconds/minute)
= 3.15 x 10 7 (31 million) seconds in a year.
(So, If you are 20 years old, then you are 620 million seconds old.)

8. The "National Debt" is the total amount of money which the United States has borrowed, and not yet payed back. This money must be payed back, eventually, by US taxpayers. Look up the current national debt, in dollars, and write this number in both scientific notation and in long form (feel free to round the number off).

The national debt currently is: 21.5 trillion dollars = \$21,500,000,000,000 = \$21.5 x 1012. (OVER Twenty Trillion dollars..and it is growing by about \$2 billion per day).

9. If you won the lottery 1000 times would you have enough money to pay off the national debt? (Note that 1 Trillion = 1,000 Billion, and 1 Billion = 1,000 Million.)

No. One thousand lottery wins of would produce:
1000 x \$1.5 billion = \$1.5 trillion which is not even 1 tenth of the national debt.

A lottery win is a large amount of money. But the national debt is a HUGE amount of money.