The Sun-Earth-Moon System(Chapter 3)

Student Learning Objectives

• Associate cycles in the sky with time

• Describe why Earth has seasons

• Determine the phase of the moon

• Compare and contrast all types of eclipses

What are the cycles that determine time keeping?

Many early calendars were based on the cycles of Moon phases.

We currently use the Gregorian calendar (1582).

o Leap years must be divisible by 4o Leap years exclude years divisible by 100

The first indications of time keeping are notched bones which may have been used to record moon phases (Africa and Europe 20,500 BC)

Practice: What is the path of the Sun through the zodiac called?

Interesting Note: 10 days were dropped from the month of October in 1582 to bring the dates for the seasons and religious holidays back into alignment when the switch was made from the Julian calendar to the Gregorian calendar.

Synodic vs Sidereal

Synodic motion relates to an Earth rotation/revolution which results in the same view of the Sun or Moon

Sidereal motion is the process of returning to the same position with respect to the background stars

http://www.skywise711.com/Skeptic/Sidereal/sidereal.html

Sidereal year:

time for Sun

to appear to return a particular place in the sky

Solar year:

time for one complete revolution (orbit) of Earth around the Sun

Precession of Time

One sidereal year = (1 + 1/26,000) tropical years

Precession makes it take longer for the same alignment between Earth and background stars each year. (20 minutes/year)

The Month is based on the motion of the Moon.

One synodic month = 29.53 days (complete cycle moon phases) One sidereal month = 27.3 days (one moon orbit)

A DayThe day is based on the rotation of Earth.

A sidereal day = 23 h, 56m, 4.09 s A solar day = 24 hours

1 → 2Earth Observer re-points to distant star (sidereal)

Earth rotates 360°

1 → 3Earth Observer re-points

to sun (synodic)

Practice

1) In general, what does sidereal time measure?

2) Why do we have leap year?

3) What is the change in position of the moon in the sky, in 24 hours? (Number of degrees)

What causes Earth to have Seasons?

The changing distance between the Earth and the Sun is not the cause of the seasons.

Our distance from the Sun only varies by 2-3%

The reason we have seasons is because Earth’s axis is tilted 23½º

The more direct sunlight a hemisphere receives, the more it is heated.

The beginning of each season is always on the same date (± 1 or 2 days).

http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html

Season Date Location of Sun

Spring Mar 22 Vernal Equinox

Summer Jun 21 Summer Solstice

Fall Sep 21 Autumnal Equinox

Winter Dec 21 Winter Solstice

Only on the date of an equinox does the Sun rise directly east and set directly west.

The altitude of the Sun changes with the seasons.

Practice1) Refer to your celestial

sphere map. Locate the position of the Sun for each season.

2) Will the seasons on Earth ever change? Explain.

3) At what latitude is the Sun directly overhead, at noon, on the first day of our “summer”? What is this latitude called?

4) At what latitude is the Sun directly overhead, at noon, on the first day of our “winter”? What is this latitude called?

Why do we see different shapes of the Moon?

As the amount of reflected sunlight that we can see changes, we see the phases of the Moon.

New Moon Phase – No illumination Full Moon Phase – All illumination 1st and 3rd Quarter – Half Gibbous – More than half Crescent – Less than half Waxing –Increasing Waning – Decreasing

Practice

1) There is always half of the Moon’s sphere reflecting sunlight.

a. True b. False

 

2) Is the moon ever visible during the day?

3) Identify the moon phase

decreasing illumination nightly

http://astro.unl.edu/naap/lps/animations/lps.swf

What are the characteristics of solar and lunar eclipses?

An eclipse is a shadow.

Umbra: complete shadow Penumbra: partial shadow

Eclipses only occur when the Earth, Moon, and Sun all lie in the same plane (the ecliptic) and are in the same location of space.

Total Solar Eclipse: umbra of the Moon's shadow touches Earth’s surface. (Sun-Moon-Earth)

In 2017, for the first time since 1918, a total solar eclipse will sweep completely across the US

Annular Solar Eclipses: anti-umbra touches Earth

Partial Solar Eclipses: penumbra touches Earth

Practice1) What is the phase of the Moon during a total

solar eclipse? 

2) The Moon is much smaller than the Sun. Why then is it possible for a solar eclipse to occur?

Lunar EclipsesTotal Lunar eclipse: umbra of Earth's shadow

covers Moon. (Sun-Earth-Moon)

The fully eclipsed Moon appears red.

Refraction

Practice1) What is the phase of the Moon during a total lunar

eclipse?

2) Why is the shadow dark and then red, during a total lunar eclipse?

3) How often can an eclipse occur?

4) Which are more commonly seen, solar or lunar eclipses? Why?