Put the Moon on trial

Everything you have read so far is a claim. Claims are cheap. This layer is about checking them.

The Moon is the single best object in the sky for a young investigator. It is bright enough to see from a city balcony, big enough that you need no equipment, and slow enough that one observation a day is plenty — but fast enough that a month of watching gives you a complete data set.

Over the next nine chapters you will run seven investigations. Some take ten seconds with a ball and a lamp. One takes a month. Each one ends with a prediction you write down before you look, because a prediction you make afterwards is not a prediction.

Keep a notebook. Date every entry. Write down what you saw, including the nights when you saw nothing.

Before you test the sky, test the explanation. If the phases really come from a half-lit ball seen at different angles, then a half-lit ball seen at different angles should reproduce every phase exactly — including the ones people find surprising.

What you need: a ball (an orange, a tennis ball, a ball of dough), a single bare lamp, and a room you can make dark.

Set up: lamp on a table at about head height, everything else switched off. Stand about two metres away. Hold the ball at arm's length in front of your face.

Here is a test you can run tonight, with no equipment at all.

Find out today's phase — from a calendar, a newspaper, a panchang, or a phone. Do not look at the sky yet.

Now, using only the rules you have learned, predict three things and write them down:

1. Shape. How much of the disc will be lit, and which edge will be bright?
2. Time. Roughly when will the Moon rise and set? (Hours behind the Sun = elongation ÷ 15.)
3. Place. At 8 pm tonight, where will it be — east, south, west, and low or high?

Then go out and check. Be strict with yourself: "roughly right" counts as right only if you said "roughly" in advance.

This is the central investigation of the topic, and the only one that cannot be rushed. It takes a month, two minutes a night.

Method. Every day, at a fixed time if you can (8 pm is a good choice), go to the same spot and record:

• Date and time, to the minute.
• A drawing. Always start by drawing a faint full circle, then shade in the part that is actually glowing. Never draw only the bright shape.
• Direction and height. East, south-east, south, south-west or west; and low, halfway, or near overhead. A clenched fist held at arm's length covers about 10° of sky — use fists above the horizon as a measure.
• Whether you saw it at all. "Not visible, clear sky, looked 8:00-8:15" is valuable data. It usually means the Moon had not risen yet, which is itself a measurement.

Stretch version. Also note the exact clock time you first see the Moon clear the same rooftop or tree each evening. That single number is the basis of Investigation 4.

Claim to be tested: the Moon rises about fifty minutes later each day.

This one you can measure yourself, to a precision that would have impressed an astronomer three hundred years ago.

Method. Choose a fixed landmark on your eastern horizon: a rooftop edge, the top of a water tank, a particular tree. Stand in exactly the same spot each time — mark it with chalk. Then, on several evenings in a row, note the exact clock time at which the Moon's upper edge clears that landmark.

The best fortnight for this is from full moon onwards, when moonrise happens at a sociable hour and slides later each night.

Result. Subtract each night's time from the next. You should get a set of numbers clustering around 50 minutes.

Claim to be tested: the Moon always shows us the same face.

This is easy to check and surprisingly satisfying, because you are testing a statement about something 384,400 km away using nothing but a pencil.

Method. On three or four nights spread across a fortnight — say a waxing crescent, first quarter, gibbous and full — draw the dark patches you can see on the bright part. Do not try to be artistic. Just get the positions right relative to the edge of the disc.

What to expect. The lit region grows from night to night, revealing more patches. But the patches you drew before do not move, do not change their spacing, and do not rotate away. The big dark oval you drew near the top edge on night four is still near the top edge on night fourteen.

Compare with what you would see if the Moon did not keep one face towards us: fresh terrain every night, with old features sliding off the edge.

Claim to be tested: the unlit part of a crescent Moon is faintly lit by light bounced off Earth.

When: two to five days after new moon, looking west about an hour after sunset. Or two to five days before new moon, looking east about an hour before sunrise.

Method:

1. Wait until the twilight has properly faded. Too early and the sky itself is brighter than the earthshine.
2. Get the bright crescent out of your direct line of sight — hide it behind a finger, a lamp-post or the edge of a roof. Glare is what defeats most people.
3. Give your eyes two or three minutes to adapt. Do not look at a phone screen during this.
4. Look at where the rest of the disc should be.

Success looks like: a complete dim circle, blue-grey, with the brilliant crescent on one edge. Once you have seen it you will never miss it again.

Claim to be tested: everyone on Earth sees the same phase at the same time; only the orientation and the clock differ.

Method. You need one contact somewhere far away — a cousin in another country, a pen-friend, a school exchange partner. If you have none, use two cities' published moonrise times and phase images.

Agree an exact moment — say 18:00 Indian Standard Time on a given date — and both describe the Moon as it appears at that instant, if it is visible.

Record: the fraction lit, which edge is bright, the height above the horizon, and the local time.

Here is a prediction with a real audience. On Karva Chauth, women who have fasted all day break the fast when they see the Moon — so the whole household wants to know when the Moon will rise, and nobody wants to be wrong.

Karva Chauth falls on chaturthi of krishna paksha: the fourth tithi after Purnima. In elongation terms the Moon has swung past full and is now between 216° and 228°, so we can take about 222° for the middle of that tithi.

Everything else follows from the rules you already have.

Science is not only collecting observations. It is asking which explanation each observation supports — and, harder, which it rules out.

Put the shadow hypothesis on trial. The claim: "The phases of the Moon are caused by Earth's shadow falling on it."

A good test is one where the two explanations predict different things. Sort the evidence below and see what survives.