ASTR 100

Lecture 5: Seasons, Moon

phases, Tides, Eclipses, and

Transits

• Seasons

• Moon phases

• Tides

• Eclipses

• Transits

A) Persephone ate six pomegranate seeds while she was in Hades’ kingdom?

B) The Earth moves in an ellipse, getting closer to the Sun in Summer?

C) The Earth’s 23.5° tilt makes days shorter in Winter, less sunlight?

D) The Earth’s 23.5° tilt means sunlight hits the Earth at a shallower angle in Winter, weaker sunlight?

Why Seasons?

Primary Reason: Stronger\Weaker Sunlight

Intensity at surface is weaker at more severe angle

because of longer path….

Same idea behind sloped armor

Notice this on a daily basis. increased

scattering by more atmosphere at Sunset

http://astro.unl.edu/classaction/animations/coordsmotion/transitmovie.html

http://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html

Tilt, shadow, and orbit in action….

Secondary (not as important) Reason:

More/Less Sunlight

South pole:

24 hr day

Hawaii:

12 hr day

If the Earth’s axis was tilted by ~90° what would the seasons feel like?

What about ~0°?

Lunar Phases

We’ll cover a lot of this in Next Week’s

exciting exercise on Lunar phases

Tidal locking.

Lunar “Day” =

Lunar “Year” =

28 days

Tidal locking of the Moon to the Earth:

The force of gravity depends on how

close matter is to the matter pulling it.

<---- <-

Over billions of

years, the

friction from

this

deformation

slowed the

Moon’s

rotation to the

length of its

orbit around

Earth.

Eclipses:

Eclipses are an alignment of the

Sun, Earth, and Moon

Solar eclipse: Moon gets in between

Earth and Sun

Lunar eclipse: Earth gets in between

Moon and Sun

Why don’t we have two a month?

Moon’s orbit is 5 deg tilted above ecliptic

Why don’t we have two a month?

Moon’s orbit does not lie on the ecliptic http://astro.unl.edu/classaction/animations/lunarcycles/mooninc.html

…An eclipse from Earth’s perspective

Shadows aren’t just shadows

Types of Lunar eclipses:

Total, partial, penumbral

Battle of Halys: May 28th, 585 B.C. …6:00 pm

Types of Solar eclipses:

Total, partial, annular

Why are lunar eclipses more frequent

than solar eclipses?

Scale model of Earth-Moon system

There was a transit of Venus 6/5/2012

Transits: “Baby solar eclipses” by a planet (an alignment of any three celestial bodies is

called a syzygy)

Key Terms:

partial/total/annular/penumbral solar/lunar eclipse,

Transit, tides, umbra, penumbra

Key Ideas:

What causes the seasons?

What causes eclipses and how are they classified?

Why causes tides?

What are transits and what needs to be true about the

transiting body?

The fact that Earth’s orbit is an

ellipse is not a reason for the seasons:

Distance to Sun Speed of Earth in orbit

Distance to Sun/variable

speed does produce effect

called the “analemma”