The size of the Moon

Perceive

Did you know that our perceptions don´t always coincide with reality?

Have you ever noticed the Moon when it is close to the horizon?
Did it look bigger than when it is high in the sky?
How much bigger?

Observe
We suggest a simple way to check your perception.

Materials you´ll need

A wooden stick 60 cm long and 1 cm wide. You can get this at your local hardware or DIY shop.
An eyehook with an inner diamter of 0.6 cm.
You´ll also need a pencil, a ruler and a small sharp nail.

1. Mark off a length 57.3 cm from one end of the stick with a pencil.
2. Make a small hole in the centre of the pencil mark with the nail.
3. Screw the eyehook into the hole as shown in the diagram.

How to use your "instrument"? You are now going to use the instrument you have just made to compare the diameter of the eyehook with that of the Moon.
	Place the end of the stick marked A close to your eye. Point the stick towards the Moon until you have it centred inside the eyehook.

These measurements should enable you to fit the Moon inside the eyehook without trouble. Since the Moon’s orbit isn’t circular, on those days when the Moon is farther from the Earth you will notice that the gap between the ring and the Moon is greater.

The observation

Choose a night of Full Moon and carry out the two observations on the same night: one with the Moon high in the sky and the other with the Moon close to the horizon.

1. Moon close to the horizon: Compare the diameters of the Moon and the ring of the eyehook.
2. Moon high in the sky: Compare once again the diameters of the Moon and the ring.
3. Do you still think that the Moon on the horizon is as big as it looked before you did this experiment?
4. If you still think that the Moon sometimes appears bigger when it is on the horizon, measure it again.

Prepare your observations:
The Moon is close to the horizon when it rises and and when it sets. For this exercise we recommend that you do the observations at moonrise, which occurs in the evening, almost at the time of sunset, when the Moon is full.

Find out the date of the next Full Moon.
Moon close to the horizon: Make your observations when the sky begins to darken. Choose an observing site with a clear view of the eastern horizon. It will help you get your bearings if you remember that the Sun sets in the west.
Moon high in the sky: make your observations a few hours after sunset.
Check for clouds. If there is any appreciable cloud you won’t be able to define the edges of the Moon correctly.

Discover
You will have found that, although we perceive the Moon to look huge when it is on the horizon, when we observe it through our instrument it fits inside the ring perfectly. Our perception that the Moon is bigger when it is on the horizon is known as the “Moon illusion”, which intrigued the ancient astronomers.

Do our senses deceive us?

Author: María C. Anguita (IAC)
English version: Terry Mahoney
Designer:Inés Bonet (IAC)
Image of the Moon:Miguel Briganti (IAC)

Send your comments

Headquarters	O.Roque						INDEX	SITE MAP
Home Page	O.Teide	A. Research	Tecnology	Training	Administration	Outreach	SEARCHER	Contact