ASTR 1010 – Homework
Assignment 1 – Spring 2009
Chapter 1
1) Question 1 – The geocentric Universe is the ancient belief that the Earth is the center of all the Universe with everything the Universe contains revolving about the stationary Earth.
This belief has been superseded in all ways. The Earth is not even at the center of our galaxy, the Milky Way. Our galaxy, in turn, has no special or unique location in the Universe.
As far as the Solar System is concerned, the geocentric model was contrasted with the heliocentric model which has the Sun at the center of the Solar System.
2) Question 2 – Galaxies are the building blocks of the Universe as far as baryonic matter is concerned. Baryons are basically heavy particles like protons and neutrons that behave ÒnormallyÓ. Astronomers currently think that most of the Universe is in an exotic, non-baryonic form of matter and energy. To find out more about these exotic formsof matter and energy, take ASTR 1020. Galaxies are composed of millions to hundreds of billions of stars - depending on the galaxy. Most of the stars are likely to have planets orbiting them.
3) Question 3 – Galaxies are moving away from each other because the expansion of the space-time fabric in which the galaxies are embedded is carrying them apart. If you work this expansion back in time, the visible Universe was once in an incredibly small and dense state. The trigger to the expansion from this state to the present day leads to the concept of the Big Bang.
4) Question 5 – The speed of light is known in physics and astronomy as ÒcÓ and is approximately 300,000 km/s. A light-year is the distance light travels in a year and is equal to
l.y. = 300,000 km/s x 60 s/min x 60 min/hr x 24 hr/day x 365.25 days/yr = 9.46 x 1012 km/yr
5) Question 11 – The astronomical unit (AU) is the average distance from the Earth to the Sun (1.5 x 108 km = 1.5 x 1011 m)
The ecliptic plane is the plane defined by the EarthÕs orbit around the Sun.
The axis tilt of a planet is the angle between the spin axis of the planet and the ecliptic plane. In the case of the Earth this axis tilt is 23.45 degrees.
6) Question 32 – The age of our Solar System is about one third the age of the Universe.
7) Question 33 – An astronomical Unit (AU) is the EarthÕs average distance from the Sun.
8) Question 40 – Stars are just too far apart from each other for there to be any real chance of a collision between them.
9) Question 46 – Mars at its closest is 56 x 106 km from the Earth. Thus it takes radio waves (that travel at c = 300,000 km/s)
56 x 106 km / 3 x 105 km/s = 1.9 x 102 s = 3.2 minutes to get there.
At its farthest (when itÕs on the other side of the Sun as the Earth), it is about 400 x 106 km away. That would take radio waves
400 x 106 km / 3 x 105 km/s = 1.3 x 103 s = 22 minutes
Pluto, on average, is 5.9 x 109 km from Earth.
That would take radio waves
5.9 x 109 km / 3 x 105 km/s = 2.0 x 104 s = 5.6 hours
10) Question 51 – To figure out the distance along the EarthÕs equator, use 2pr for the circumference, where r is the radius (look at Appendix E).
Circumference of Earth is 6378 km, so the circumference (the distance along the EarthÕs equator) is about 40,000 km. At 100 km/hr, it would take 400 hours to go completely around the equator.
Distance from Sun to Earth is 1 AU or 1.5 x 108 km. At 100 km/hr that would take 1.5 x 106 hours or about 170 years.
We saw in the question above that Pluto is about 5.9 x 109 km from Earth.
So, at 100 km/hr, thatÕs 5.9 x 107 hr or over 6700 years.
The nearest star is 4.4 l.y. away, which is 4.2 x 1013 km. At 100 km/hr, thatÕs 4.2 x 1011 hours or 1.1 billion years!
11) Question 52 – We saw above that Alpha Centauri is 4.2 x 1013 km away. If you wanted to get there in 100 years, you would have to go 1.3 x 104 km/s or 4.8 x 107 km/hr. This is about 4% of the speed of light. ItÕs also almost a thousand times faster than our current fastest spacecraft.