Astronomy 115: Introduction to Astronomy
San Francisco State University

Homework Solutions #1

  1. The Speed of the Earth 30,000 km/s. Express this number in km/s and miles per second.

    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:

    (1km /1000 m) = 1

    Hence, 30,000 m/s *( 1km/1000 m) = 30 km/s

    Similarly, since 1km = 0.6 miles

    30 km/s * (0.6 miles / km ) = 18.6 miles per second

  2. There aren't any months in 3 light years, because a light year is a measure of distance, while a month is a measure of time. The question makes no sense, like saying, "How many inches until Friday?"

  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 number of seconds in a year is: 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. If you are 20 years old then you are 620 million seconds old.

  7. 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: 20 trillion dollars = $20,000,000,000,000 = $20.0 x 1012. (Around Twenty Trillion dollars..and it is growing by about $2 billion per day).

  8. 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.

    Additional information:

  9. $250 billion/$7.5 billion = 250/7.5 = 33.3.

    The U.S. spends about 33 times more on interest paying off the national debt than on the NSF.