Astronomy

Class Notes

Jim Mims

Chapter 1Fundamentals

Our Place in Space

•Scales are very large: measure in light-years, the distance light travels in a year – about 10 trillion miles

Our Place in Space

• This galaxy is about 100,000 light-years across:

Scientific Theory and the Scientific Method

Scientific theories:

• must be testable

• must be continually tested

• should be simple

Scientific theories can be proven wrong, but they can never be proven right with 100% certainty

Scientific Theory and the Scientific Method

• Observation leads to theory explaining it

• Theory leads to predictions consistent with previous observations

• Predictions of new phenomena are observed. If the observations agree with the prediction, more predictions can be made. If not, a new theory can be made.

The “Obvious” View

Simplest observation: look at the night sky

About 3000 stars visible at any one time; distributed randomly but human brain tends to find patterns

The “Obvious” View

Group stars into constellations: figures having meaning to those doing the grouping

Useful: Polaris, which is almost due north

Not so useful: Astrology, which makes predictions about individuals based on the star patterns at their birth

The “Obvious” View

Stars that appear close in the sky may not actually be close in space:

The “Obvious” View

The celestial sphere:

Stars seem to be on the inner surface of a sphere surrounding the Earth

They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects

Angular Measure

• full circle contains 360° (degrees)

• each degree contains 60′ (arc minutes)

• each arc minute contains 60′′ (arc seconds)

• angular size of an object depends on actual size and distance away

Latitude and Longitude

Latitude is measured from the equator, with positive values going north and negative values going south. Longitude is measured from the Prime Meridian (which is the longitude that runs through Greenwich, England), with positive values going east and negative values going west. So, for example, 65 degrees west longitude, 45 degrees north latitude is -65 degrees longitude, +45 degrees latitude.

Prime Meridian

The Prime Meridian is the meridian (line of longitude) at which longitude is defined to be 0°.

The Prime Meridian and the opposite 180th meridian, at 180° longitude, which the international date line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the equator 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian passes through the Royal Observatory, Greenwich, in east London, United Kingdom, known as the International Meridian or Greenwich Meridian.

Celestial Coordinates

• Declination: degrees north or south of celestial equator

• Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

Earth’s Orbital Motion

Seasonal changes to night sky are due to Earth’s motion around Sun

Earth’s Orbital Motion12 constellations that Sun moves through during the year are called the zodiac; path is ecliptic

Elicptic

The plane of the ecliptic (also known as the ecliptic plane) is the plane of the Earth’s orbit about the Sun.

It is the primary reference plane when describing the position of bodies in the Solar System, with celestial latitude being measured relative to the ecliptic plane.

In the course of a year, the Sun's apparent path through the sky lies in this plane. The planetary bodies of our Solar System all tend to lie near this plane, since they were formed from the Sun's spinning, flattened, protoplanetary disk.

The Ecliptic Plane was so named because a Solar eclipsse can only occur when the Moon crosses this plane.

Earth’s Orbital Motion

Earth’s Orbital Motion

Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years

The Measurement of Distance

Triangulation: measure baseline and angles, can calculate distance

The Measurement of Distance

Parallax: similar to triangulation, but look at apparent motion of object against distant background from two vantage points

The Measurement of DistanceMeasuring Earth’s radius:

Done by Eratosthenes about 2300 years ago; noticed that when Sun was directly overhead in one city, it was at an angle in

another.

Measuring that angle and the distance between the cities gives the radius.

Measuring Distances with Geometry

Converting baselines and parallaxes into distances:

Measuring Distances with Geometry

Converting angular diameter and distance into size: