Astronomy Notes

Part 1: Terms and Basics

Dr. John P. Pratt
Updated 7 Sep 2004

The Scientific Method

Science is based on the "Scientific Method." It has three steps which are usually repeated many times in succession. These steps are:

  1. Make Observations. The observations are often called "facts", but that doesn't mean they are always correct.
  2. Propose a Theory. A theory or hypothesis is a proposed explanation of the observations.
  3. Use the Theory to Predict Future Observations.

Note that while a scientific theory can be disproven by any one experiment, it can never be proven true, because a new falsification experiment might prove it false. Thus, all scientific results are provisional and not believed to be absolutely true.

Truth is how things "really are," and so truth is entirely outside of the realm of science. Truth is usually hidden, such as a person's motives or ultimate causes. Truth is the subject of religion, philosophical speculation and courtrooms, but not of science.

Daily Application. When anyone tells you anything, try to separate the facts (observations) from the theories (explanations). Example: "The stock market fell 300 points today because of Mid-east tension." The fact is the stock market fell, the theory is it was caused by Mid-east concerns, and the truth of what "really" governs the stock market is probably not known to anyone.

Introduction to Astronomy

The universe usually means "everything that exists." In astronomy we study the observable part of the universe.

Powers of 10 are used everywhere in astronomy, so become very familiar with them.

The SI metric system, which our text uses, has three basic units.

The metric system has units in powers of ten for ease in computations, with prefixes

The Pointers are separated by 5°.
Angular Measure is used to measure arc lengths in the sky.

Angular Measure is also used to measure arc lengths on the spherical earth.

Astronomical Terms Describing Sky's Motion

Even though we now know that the earth is spherical, rotates daily on its axis, and revolves around the sun, the best way to actually understand the motion of the starry sky is think of yourself as at the center of a circular flat earth, with the sun, moon, planets and stars all at the same distance, as if attached to the inside of a planetarium dome. With that in mind, you need to know the following terms:

The Celestial Sphere: A simplified model of the sky, where all the stars are considered to be the same distance away from us, on the inside surface of a sphere, called the "celestial sphere."
Zenith: The point on the celestial sphere exactly overhead.
Meridian: The North-South semi-circle on the celestial sphere from the north point on the horizon, passing through the zenith to the south point on the horizon.

Altitude & Azimuth
Because we are ignoring the distances to the stars, only two coordinates (numbers) are used to describe exactly where a star in the sky is (see illustration):

The north celestial pole is the point in the sky above the earth's north pole.

The North Star (Polaris), is a star that is within 1° of the north celestial pole.

The celestial equator is the circle in the sky which is 90° away from the north celestialpole.

The ecliptic is the apparent path of the sun through the sky as the earth orbits the sun.

Constellations are the ancient figures which form a pictorial star map.

Observing the Sun

The Sun Moves Eastward through the zodiac 1° daily

The Seasonal Path of the Sun.
Seasons are caused because the earth's rotation axis is tilted 23 1/2° to the ecliptic.


The Analemma
The Analemma

Our solar calendar is designed to stay aligned with the seasons.

Observing the Moon

The moon's phases are caused because as the moon orbits the earth, we can only see part of the half of the moon that the sun lights up.

The crescent moon correctly indicates about 4 a.m.
The bright part of the moon is always lit by the sun, so draw it that way in pictures. For example, a crescent moon in a dark sky must be tipped to where the sun would be below the horizon.

The moon rises about 50 minutes later every night, and it moves 13° westward through the stars nightly.

A lunisolar calendar tracks both the sun and the moon.

Eclipses occur when the sun and moon align and one is in the other one's shadow.