☰ Contents · Astronomy

The Moon, calendars and eclipses

Lessons 15–17 · 3 lessons · M. Mamadazimov. Astronomy Grade 11, 1st edition. DAVR NASHRIYOTI, Tashkent, 2018
15

The Moon’s motion, phases and periods

Textbook: pp. 25–26
GoalExplain the Moon’s motion, phases and periods (sidereal and synodic month) and calculate with them.
New words
lunar phases · Oy fazalarisynodic month · sinodik oysidereal month · yulduz oyinew Moon · yangi oy
Explanation

The Moon revolves round the Earth from west to east at a mean distance of about 384,400 km; its orbit is inclined ≈ 5° to the ecliptic. One full turn relative to the stars takes 27.32 days (the sidereal month), while the same phase repeats every 29.53 days (the synodic month), because in that time the Earth also moves around the Sun. The Moon does not emit light but reflects sunlight; the part of the lit half that we see from Earth is the phase: new Moon, waxing crescent, first quarter, full Moon, waning, last quarter. The Moon rotates on its axis in the same period as it orbits, so one side always faces the Earth. Each day the Moon shifts ≈ 13° eastward against the stars, so it rises on average ≈ 50 minutes later every day.

Sunlightnew Moonfirst quarterfull Moonlast quarterEarthnot to scaleshaded – dark half
Worked examples
If the Moon rises today at 20:00, six days later it rises ≈ 50 · 6 = 300 minutes = 5 hours later, i.e. about 01:00.
Period
Sidereal month27.32 days
Synodic month29.53 days
Difference of synodic and sidereal months: 29.53 – 27.32 = 2.21 days; the Earth moves ≈ 27° in that time, so the Moon needs about 2 more days to catch up.
Class activity

Over a month, with an adult, draw the Moon’s shape at the same time each night it is visible and make a phase calendar. The Moon needs no special filter, but go outside in the evening only with an adult.

Practice
1
What is the synodic month and how long is it?
2
If the Moon rises about 50 minutes later each day, how many minutes later is it after 4 days?
3
Why do we see only one side of the Moon?
4
In which phase is the Moon between the Earth and the Sun, and why can we not see it?