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panda.unm.edu/courses/sanfratello/astro101

Remember - Read syllabus and schedule(contains textbook reading assignments andsuggested problems from the text.)

The Constellation Orion

The "Solar Day" and the "Sidereal Day"

Solar Day

How long it takes for the Sun to return to the same position in the sky (24 hours).

Sidereal Day

How long it takes for the Earth to rotate 360o on its axis.

These are not the same! Which is longer? Why?

(Demo)

One solar day later, the Earth has rotated slightly more than 360o .A solar day is longer than a sidereal day by 3.9 minutes(24 hours vs. 23 hours 56 minutes 4.091 seconds).

Difference due to rotation and revolution of Earth!

Q: What causes the seasons?

Seasons

Seasons

Play Movie

Summer Winter

In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler

Seasons caused by tilt of Earth’s axis of rotation!

Earth is farthest from sun during summer!

Precession

What causes precession? How long does one cycle take?

Precession

The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis.

Earth

Moon

Spin axis

**Vega Polaris

Precession Period 26,000 years!

Triangulation - Using Geometry to Measure Distances

• Know: Angle at A Angle at B Length of Baseline

• Calculate: Distance to object

distance =baseline*tan(B)

Parallax

• Triangulation - Measure angles at points A and B

• Parallax - Know Baseline. Measure third angle in triangle made by A, B, and object in space

Baseline problem

Parallax Geometry

Ex. “Stationary” moon in moving automobileParallax angle is inversely proportional to distance of object

Close object causes large parallax

Distant object causes small parallax

How are the Earth, Moon, Sun, and stars laid out in space?

What lies at the “center”? Motion(s) of the Earth? Motion(s) of the Moon? What about the stars?

The Motion of the MoonThe Moon has a cycle of "phases", which lasts about 29 days.

Which way is the Sun here?

Moon moves in synchronous motion – it rotates on its axis in exactly the same amount of time it takes to orbit earth.

Why doesn't the earth prevent the sunlight from reaching the moon during its “Full phase”?

Eclipses

What sort of alignment of the Earth, Moon, and Sun is required for a lunar eclipse?

How about for a solar eclipse?

Eclipses

Lunar Eclipse (around 100 minutes)

When the Earth passes directly between the Sun and the Moon.

Sun Earth Moon

Solar Eclipse (no more than about 8 minutes)

When the Moon passes directly between the Sun and the Earth.

Sun EarthMoon

Lunar Eclipse

Total Solar Eclipse

During which phase(s) can a lunar eclipse occur?

What about a solar eclipse?

Why don't eclipses occur every month?

Moon's orbit tilted compared to Earth-Sun orbital plane:

SunEarth

Moon

Side view

5.2o