Discovering the Solar System

Discovering the solar system: projects for the keen–eyed and camera–toting observer

The Big Moon” Illusion
a naked eye and camera activity

Casual skywatchers since the time of the ancient Greeks have seen the rising moon as appearing much larger than after it has climbed higher in the sky. The moon is often portrayed in film and television as being very large and bright when it is near the horizon. All this flies in the face of the fact that the actual apparent size of the moon, whether it is rising or it is at its highest point in the sky is quite small. Fully extend your arm and outstretch your hand. The moon’s apparent diameter is only about 1/4 the width of your index finger.

The common explanation of the “Big Moon” illusion is that when the moon’s apparent size is compared to familiar landscape objects, such as distant houses and trees, our mind interprets the moon as being quite large. Then, when it moves higher in the sky, there are no nearby comparison objects. The moon’s apparent size then appears to shrink, and it seems to lie much farther away. While sounding plausible, this reasoning does not explain why the same effect occurs at the beach when the moon is seen hovering just above a flat, featureless ocean horizon, or in the desert when the moon is cast against sweeping sand formations. Studies have sought a deeper psychological explanation.


See the big moon illusion for yourself on the evening of April 26th, 27th or 30th. From a location that has a low horizon line, look to the east at sunset for the rising moon, or, on the following morning, to the west before sunrise.

  1.  Isolate the moon by viewing it through a narrow tube, such as a drinking straw. Note its size compared to the tube’s field of view. Wait two hours or more and repeat the observation. (If it is a morning observation, first look a couple of hours before sunrise.)
  2. Use a digital camera at full optical zoom and take an image of the rising moon. Be sure the camera is properly focused and that the image is not overexposed. Again, wait a couple of hours, then take another image. Download both images on a computer and view them at the same image scale.


Are the moon sizes the same?

Compare the size difference between moon at apogee and perigee
a camera activity

A direct comparison between the apparent sizes of the moon when it is near perigee (the moon’s closest point to Earth) and when it reaches apogee (the moon’s farthest point) can be made. Apogee occurs about April 14 when the moon is in a waxing crescent phase, low in the west after sunset.

Simply take a digital photo of the moon on either the 14th or 15th, shortly after sunset. Take another image in the evening of perigee, April 27, after moonrise when it is a thick gibbous phases. Use the camera’s full optical zoom feature, and make sure the lens is properly focused. (Try using a manual focus set on infinity.) Be careful not to overexpose the images.

Directly compare the apogee and perigee moon sizes on a computer using the same image scale. The April 27 image will be found to be about 10% larger than the April 14 image.

Discover lunar libration, seeing the far side of the moon
a binocular and camera activity

One interesting consequence of the moon’s elliptical orbit is the phenomenon known as libration. The moon presents the same hemisphere towards Earth as it orbits our planet. Therefore, we always see its same side; we never see its far side. Strangely though, during each month, we are able to observe about 59% of the lunar surface.

The moon traces an elliptical path around Earth. One of the features of a body moving in an elliptical orbit is that, when it is nearer to the parent body, it moves faster, and when it is farther, it moves slower. Therefore, the moon moves slowest at apogee and fastest at perigee. All the while, the moon rotates at a constant rate, completing one full rotation in every lunar orbit. As a result of these two factors – the changing speed of the moon in its orbital path and its constant rotational rate — plus the changing curvature of its elliptical path, observers on Earth are able, at times, to see slightly around the western limb of the moon, and, at other times, to see slightly around the eastern limb. This is an east-west libration.

There is also a north-south component, because, at times, the moon is either slightly above or below the ecliptic, permitting observers on Earth to see slightly over the moon’s south or north polar regions, respectively.


Activity for binoculars or a small telescope: Observe Mare Crisium near the moon’s eastern edge. (The moon’s eastern edge is the side that appears farthest west in our sky.) It has a dark floor, making for easy identification. Photograph it using a digital camera at full optical zoom or sketch it noting how far Crisium lies from the rim of the moon. Be sure to focus the camera and be careful not to overexpose the image. Do this on April 20, and do so again on April 25. Download the images on a computer displaying the same image scale. Closely examine the amount of lunar surface between the eastern edge and Mare Crisium. The April 25 image should show much less distance than the April 20 image.

See the accompanying Libration Crisium diagram.

Click on the image to view/print the pdf

From April 25, 21:00 UT to April 26, 3:00 UT, the crescent moon moves in front of the northern portion of the Hyades star cluster.

Observe the event through binoculars, and capture it with a digital camera. Note the time for each photo. As the minutes pass, the dark edge of the moon slowly moves in front of different stars, blocking them from view. Other stars pop out the moon’s brightly lit crescent side. This demonstrates that, even though the moon appears to move westward in our sky, it also travels eastward in its orbit around our planet. It moves about one of its diameters every hour.


Sunrise and sunset locations
a naked eye activity

It surprises many people that the sun doesn’t rise or set at the same location on the eastern and western horizons throughout the year. Earth’s rotational axis is tilted resulting in the sun rising the farthest south on Dec. 21 and the farthest north six months later on June 21.

On one of the first days of April, note where the sun peeks above the eastern horizon and where it sets along the western horizon. Repeat the observations from the same location on a day near the end of the month. (Do not look at the sun! When it first begins to rise, stopping looking. After its last rays disappear below the western horizon, make your evening observation.)

A method for observing lunar detail by the unaided eye
a naked eye activity

The full Moon is very bright, so bright that lunar detail is overwhelmed by the lunar glare. Here is an easy way to see more.

  1. Drill a 1/16-inch (or 1.5 mm) diameter hole in a plastic soft drink bottle cap (or other opaque thin plastic). Make sure it is an unobstructed, round hole.

Look through the hole at the bright Moon. The image brightness will be much dimmer than normal – over 90% dimmer – reducing or eliminating any lunar glare. The image should also be much sharper because the bottle cap blocks light from entering the outer portion of your pupil, where imperfections of the eye’s curving optical path likely lie. 

Many observers are able to easily able to see crisp edges of the lunar maria and can see the changing effects of lunar libration.