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 4 o 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: 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

 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 = days (complete cycle moon phases)  One sidereal month = 27.3 days (one moon orbit)

A Day  The day is based on the rotation of Earth.  A sidereal day = 23 h, 56m, 4.09 s  A solar day = 24 hours 1 → 2 Earth Observer re-points to distant star (sidereal) Earth rotates 360° 1 → 3 Earth 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).

 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.

Practice 1)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  1 st and 3 rd 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

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

Practice 1)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 Eclipses  Total Lunar eclipse: umbra of Earth's shadow covers Moon. (Sun-Earth-Moon)

 The fully eclipsed Moon appears red. Refraction

Practice 1)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?