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Observing and the Sky
Astronomy 311
Professor Lee Carkner
Lecture 3
Our Point of View

Think about the motions of the heavens
from different points of view
Remember that everything is moving at
once
The Changing Sky

Sky makes one complete rotation per
24 hours (Diurnal motion)

Sky also makes one complete rotation
per year (Annual motion)
Caused by the motion of the Earth around
the Sun
Diurnal Motion
Annual Motion
The Observer’s View
Observing
Can measure distance on the sky in
degrees (360 degrees = complete circle)
Horizon -Zenith -Meridian -- line running from north to
south through zenith
The Solar Year
The Seasons

When the northern hemisphere is pointed
at the Sun, it is summer here

Seasons are not caused by Earth being
more or less distant from the Sun!!!
Direct and Indirect Sunlight
Solstice
Equinox


When the Sun is
highest or lowest in
the sky

When the Sun is
overhead at the
equator

Longest or shortest
day of year
Day and night equal
in length
Changing Day Length
Lines on a Globe
Equator -Tropic of Capricorn -- 23 1/2 degrees
south of the equator, Sun overhead on
Winter solstice
Tropic of Cancer --
Arctic and Antarctic Circles -- 23 1/2
degrees south or north of the pole, Sun
never sets or rises at solstice time


Equal to the angle of Polaris
above horizon

Longitude (degrees East or
West of the Prime Meridian
running through Greenwich
England)
Navigation

The Celestial Sphere
Project the lines on a
globe into space to
form a coordinate
system
North Celestial Pole --
Celestial Equator -Project the Earth’s
equator into space
Celestial Coordinates
Right Ascension -- degrees east from where
the sun is on the Vernal Equinox (first day of
spring)
Declination --
The coordinates do not move or vary with
location on the Earth, they are fixed to the
stars
The Ecliptic
Most of the objects in the solar system
have their orbits in the same plane,
called the ecliptic

The ecliptic passes through 12
constellations known as the zodiac
Constellations of the Zodiac
Precession
The Earth “wobbles” as it spins,
causing the Earth’s axis to point at
different parts of the sky

This changes where the equinoxes are
in the sky
Astrology

Your sign is supposed to be where the
Sun was on the zodiac when you were
born

For example: Sept 10th = Virgo, but Sun is in
Leo. All star signs are about 1 month off
Science and Pseudoscience
Astronomy is a science, it tries to form a
picture of the universe based on observation
and reason. It is subject to proof

Astrology is a pseudoscience, it uses some of
the terminology of science, but its basic tenets
are not subject to proof

Next Time
Meet back in Science 102 on Monday
Read Chapter 1.6