1. Ch. 30.3 The Lunar Cycle

2.  All moonlight is reflected light.  The different parts of the lighted side of the moon that face the earth are known as the phases of the moon.

3. Phases of the Moon  New Moon—the moon is between the earth and the sun; no lighted areas of moon visible from earth.  When the size of the visible moon is increasing, it is a waxing moon.  Waxing-Crescent—when a sliver of the moon becomes visible from earth.

5.  First-Quarter—moon has completed a quarter of its orbit around earth, and half of its lighted surface is visible from earth.  Waxing-Gibbous—visible portion of the moon is larger than a semicircle and still increasing.  Full Moon—earth is now between sun and moon, and moon appears as a full circle.

7.  After full moon, portion of moon visible from earth decreases. It is called a waning moon.  Waning-Gibbous—visible portion larger than a semicircle, but size is decreasing.  Last Quarter—when visible portion becomes a semicircle.

9.  Waning-Crescent—when the visible portion is only a sliver, before the new moon phase.  During the crescent phases, the unlit portion of the moon can be dimly seen due to light reflected off of the earth—earthshine.

11.  29.5 days for moon to go through a complete cycle of phases…longer than its rotation of 27.3 days, because earth is orbiting sun, and extra 2.2 days needed for moon to travel extra distance to again get exactly between the earth and sun.

12. The Calendar  Calendars are systems developed by societies for measuring the passage of time.  Calendars have 3 basic units…  Day—earth rotates once on its axis.  Month—Moon goes through one cycle of phases. 29.5 days.  Solar year—earth makes one orbit around the sun. 365.24 days.

13.  Problem—none of the units divide evenly into each other. Calendars must account for this.

14. The Julian Calendar  First calendar in Rome had 304 days divided into 10 months.  Julius Caesar’s astronomers later created a 12 month calendar, with 11 months of 30 or 31 days, and February with 29 days.  Later, Feb. day moved to Aug., and every 4 th year was leap year…365.25 day year…11 minutes too long.

15.  Julian calendar used for over 1500 years.

16. The Gregorian Calendar  By 1580 Julian calendar was 10 days ahead.  Pope Gregory XIII had 10 days dropped from October in 1582.  Revised calendar has 3 fewer days every 400 years…no leap year in years ending in 00 that aren’t divisible by four.

17.  This Gregorian Calendar is used by most of the world.  The calendar year is only 26 seconds longer than the solar year.  Less than 1 day error every 3000 years.