Discovering the Solar System

Discovering the solar system: projects for the keen-eyed, camera-toting, and binocular wielding 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 22nd or 23rd. 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 the moon at apogee and perigee
a camera activity

A direct comparison can be made 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). Perigee occurs on April 7, 2024, the day before new moon. Apogee falls on April 20, just after first quarter when the moon lies high in the south at sunset.

Simply take a digital photo of the moon on either the 10th or 11th, shortly after sunset. Take another image in the evening of April 20, 2024, just after sunset when it is a thick gibbous. 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 20 image will be found to be about 10% smaller than the April 10 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. Why?

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 13 or 14, and do so again on April 21 or 22. 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 13 image should show much less distance than the April 21 image. 

See the accompanying Libration Crisium diagram.

Click on the image to view/print the pdf.

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, stop 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 

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. 

2. Hold the cap close to your eye and look through the hole at the bright, near full moon. 

The image brightness of the moon 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 most imperfections of the eye’s curving optical path likely lie. 

Many observers easily see crisp edges of the lunar maria and even see the changing effects of lunar libration.


Earth’s shadow – it is visible every night
a naked-eye activity

Find out when sunset is at your location. At that time, look along the eastern horizon directly opposite the setting sun.

You will likely first see a reddish-orange glow along the eastern horizon. That is the “Belt of Venus.” It is caused by back scattered sunlight off particulates in our planet’s atmosphere.

After 2 minutes, a dark band barely clears the horizon. After another 2 or 3 minutes, the band is seen to be a dark arc rising higher each minute.

That band is the shadow of Earth!

After 10 minutes after sunset, the shadow has risen about 10º in the eastern sky and its leading edge is beginning to blend with the darkening sky above. In another few minutes, the shadow seems to fade then disappear.

See the accompanying Alpenglow/Belt/Shadow diagram.

Click on the image to view/print the pdf.

Zodiacal Light
a naked-eye activity

Look to the west 90 minutes after sunset for a subtle glow in the form of a narrow cone stretching nearly 45º vertically from the horizon. To fully appreciate it, you must be positioned away from city lights and the atmosphere must be clear.

This glow is thought to be caused by dust concentrated in the plane of the solar system near the orbits of Mars and Jupiter.

As the night proceeds, the tip of the cone drops closer to the western horizon, fully setting 2-1/2 to 3 hours after sunset.

See the accompanying Zodiacal Light diagram.

Click on the image to view/print the pdf.