Moon phases
02: Moon phases
Starts with a question you can answer by looking up: what phase is the Moon in right now? Ends with you having built the calculation yourself and understanding why two reasonable models disagree.
Lessons
| # | Lesson | What you get |
|---|---|---|
| 1 | Go and look | An observation log, started today |
| 2 | Why phases happen | The geometry, with a lamp and a ball |
| 3 | Calculating a phase | Both models, built up from nothing |
| 4 | Synodic and sidereal | Where the extra two days come from |
Prerequisites
Module 01, or at least a working install and a saved location.
The observation gate
Lesson 1 asks you to start a log and add to it over about a month. Start it now and keep reading; do not wait. The calculation lessons are much more interesting when you have your own data to check them against, and a lunar cycle takes as long as it takes.
Cloudy for a week? Note the gap and carry on. Missing data is data.
What you will be able to do
- Say what phase the Moon is in without looking it up
- Predict where and when to see it
- Compute the phase for any date, two different ways
- Explain why the simple model drifts, and by how much
- Explain the 29.5 versus 27.3 day puzzle from first principles