Observing and the Sky

Astronomy 311Professor Lee

CarknerLecture 3

Our Point of View

We are all standing on different parts of a large spinning ball moving through space Remember that everything is moving

at once

The Changing Sky North Pole of the Earth is pointed at

Polaris (the North or Pole star), which stays stationary as the other stars move around it

Sky makes one complete rotation per 24 hours (Diurnal motion)

Sky also makes one complete rotation per year (Annual motion)

Diurnal Motion

Annual Motion

The Observer’s View

Observing

Can measure distance on the sky in degrees (360 degrees = complete circle)

Horizon -- Zenith -- Meridian --

The Solar Year

The Seasons

Tilt of Earth’s axis causes seasons

Sunlight is more direct in the summer

Direct and Indirect Sunlight

Solstice and Equinox Solstice

When the Sun is highest or lowest in

the sky Equinox

When the Sun is overhead at the

equator

Times of year

Vernal Equinox -- March First day of spring

Summer Solstice -- June First day of

summer

Autumnal Equinox --September First day of

autumn

Winter Solstice -December First day of winter

Changing Day Length

Lines on a Globe Equator -- Sun is overhead at

equinox Tropic of Capricorn -- 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

Navigation

Latitude (degrees North of the Equator) Only works in Northern

hemisphere Longitude (degrees East

or West of the Prime Meridian running through Greenwich England)

The Celestial Sphere

Project the lines on a globe into space to form a coordinate system

North Celestial Pole –

Celestial Equator --

Celestial Coordinates

Right Ascension --

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

The belief that the positions of heavenly bodies at the time of your birth foretells your future

Your sign is where the Sun was on the zodiac when you were born

For example: Sept 13th = 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

Next Time

Meet back in Science 102 Read Chapter 1.6