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Chapter 26 – Studying Space
The study of the universe is called astronomy.
A collection of stars, gas, and dust bound together by gravity is called a galaxy. The galaxy we
live in is called the Milky Way.
The long distances in space are measured in units called light-years and AU’s. A light-year is the
distance light travels in one year. Light travels at a rate of 300,000,000 meters each second. In
a year light travels (365 days per year)(24 hours per day)(60 minutes per hour)(60 seconds per
minute)(300,000,000 meters per second) which equals 9,460,800,000,000,000 meters. An AU is
an Astronomical Unit, which is the average distance between the Earth and the Sun. This
distance is about 150,000,000 kilometers.
The two types of optical telescopes are reflecting (which uses mirrors) and refracting (which
uses lenses). A mirror only needs one perfect side, while lenses need two perfect sides.
Therefore, most inexpensive beginner telescopes are reflectors. An optical telescope is judged
on its ability to gather light rather than its ability to magnify. There are other telescopes that
use wavelengths not in the visible spectrum and these telescopes detect radio waves, gamma
rays, x-rays, or infrared.
Telescopes beyond the atmosphere of the Earth send back much more detailed information.
This space-based astronomy includes the Hubble Space Telescope, as well as the Voyager,
Galileo, and Cassini spacecraft.
We describe planetary movements in terms of the planet’s rotation (the time it takes to travel
once around on its axis), and its revolution (the time it to travel once around the Sun). Another
name for rotation is spin, and another name for revolution is orbit. Planets orbits are not
perfect circles, as we will discuss in chapter 27. The point where the planet is closest to the Sun
is called perihelion, and the point farthest from the Sun is called aphelion.
A complete rotation for the Earth takes approximately 24 hours. Since there are 3600 in a circle,
each of the 24 time zones are about 150 of longitude across. As you go east, time will increase
(1pm, 2pm, 3pm, . . .). As you go west, time will be less (3pm, 2pm, 1pm, . . .) The zero degree
line of longitude is called the Prime Meridian. The 1800 line of longitude is called the
International Date Line. Date changes just the opposite of our time changes. For example, as
you cross the date line going from California to Japan, the date will advance a day.
The Earth’s axis is tilted at 23.50. When the northern hemisphere is pointing toward the Sun, we
experience summer, and when our hemisphere points away from the Sun, we experience
winter. On June 21st, when the Sun is directly overhead at 23.50 N latitude (called the Tropic of
Cancer), we have our summer solstice. On this date the Sun rises and sets farthest South, the
Sun is highest in the sky, and the path of the Sun above the horizon is longest (our longest day).
On December 21st when the Sun is directly overhead at 23.50 S latitude (called the Tropic of
Capricorn), we have our winter solstice. On this date the Sun rises and sets farthest North, the
Sun is lowest in the sky, and the path of the Sun above the horizon is shortest (our shortest
day). On March 21st and September 23rd, when the Sun is directly overhead at the equator, we
have our vernal (spring) and autumnal (fall) equinoxes.