Astro 100 - Chapter 2 | Hunter College

Why do seasons exist?

Earth’s axis tilt

How are seasons determined?

Position and axis tilt of Earth when orbiting the Sun

Four Special Points of Season Progression

Winter Solstice (December)

Spring Equinox (March)

Summer Solstice (June)

Fall Equinox (September)

Sun’s path during the Summer Solstice

Highest path; rise and sets at most extreme north due east

Sun’s path during the Winter Solstice

Lowest path; rise and sets at most extreme south due east

Sun’s path during equinoxes

Sun precisely rises due east and sets due west

How much sunlight does the Northern Hemisphere get during each solstice and equinox?

Summer Solstice: Most sunlight

Spring & Fall Equinox: Equal sunlight on both hemispheres

Winter Solstice: Least sunlight

Why do lunar phases occur?

Moon’s orbit around Earth

What is the moon illuminated by?

Half from the Sun and half is dark

How long is the moon phase cycle?

29.5 days

Phases of the Moon (Waxing)

New → Crescent → First Quarter → Gibbous → Full

Visible during afternoon/evening

Reveals itself more and rises later each day

Phases of the Moon (Waning)

Full → Gibbous → Third Quarter → Crescent

Visible during late night/morning

Starts disappearing and sets later each day

Lunar Eclipse

Earth’s shadow falls on the moon, giving it a reddish color

Can only occur during full moon

Can be Penumbral, Partial, or Total

Solar Eclipse

Moon’s shadow falls on the Earth

Can only occur during new moon

Can be Total, Partial, or Annular

Constellation

A region of the sky

How many constellations are there?

88 constellations

Celestial Sphere

A sphere where all the constellations lie at but at different distances

Ecliptic

Sun’s apparent path throughout the celestial sphere

Location of north and south celestial pole

North celestial pole: Above Earth’s North Pole

South celestial pole: Above Earth’s South Pole

The Milky Way

A wave of light circling the celestial sphere

How to know the location of an object in your local sky?

Its altitude and direction

Zenith

Directly above you(90 degrees)

Horizon

All points that are 90 degrees away from the zenith

Meridian

A line that passes through the Zenith and connects the North and South poles

Angular Measurements

Full Circle = 360 degrees

1 degree = 60’ arcminutes

1’ arcminute = 60” arcseconds

Visibility of Stars

Stars in the north celestial pole are circumpolar and never set

Cannot see stars in the south celestial pole

All other stars rise east and set west

Why do we see different constellations?

Position of Earth and the time of year

Ex: Sagittarius - Summer / Orion - Winter

(RAHHHHHH Beyblade taught me all these constellations)

What planets were known during ancient times?

Mercury - Most difficult to see, always near the sun in the SKY

Venus - Very bright when visible; morning/evening “star”

Mars - Noticeably red

Jupiter - Very bright

Saturn - Moderately Bright

Planetary Motion

When planets usually move a little eastward every night

Apparent Retrograde Motion

When the stars move westwards for a few weeks

Stellar Parallax

Using the Earth’s orbit(every 6 months) and distant “fixed” stars to determine the position of a nearby star

Ancient Greek’s conclusion on observable parallax

Earth does not orbit the sun; it is the center of the universe

Explanation: They didn’t think the stars could be THAT far away, meaning they tried using stellar parallax with the naked eye but couldn’t find any difference in star positions, hence the making of Earth-centered systems.