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Astronomy
Class Notes
Jim Mims
Chapter 1
Fundamentals
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 Motion
12 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 Distance
Measuring 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: