April 10- 28, 2017

Three lunar challenges in one! Learn more about the Moon.

Every GAM, we run a series of Observing Challenges, developed by The Astronomical League. Whether you are a complete novice to astronomy or a seasoned veteran there will be something for you! Some of the challenges can be done in a night and some will take the whole month. 

If you are a sidewalk astronomer or part of an astronomy club you might like to run events to help people complete the challenges. If are planning an Observing Challenge event, don't forget to register your event!

Discuss your observations, ask and answer questions and meet fellow astronomers working on these challenges from around the world in our forum.

Share your photos from this challenge with us and the world on Facebook, or Tweet using #GAM2017 hashtag (@gam_awb). 

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. Extend your arm and outstretch your hand. The Moon’s apparent diameter is 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. However, 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 the full Moon, April 10, or the next evening, April 11. From a location that has a low horizon line, look to the east at sunset for the rising Moon.

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.

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 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.

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. Perigee occurs on April 27 when the moon exhibits a thin crescent in the west just after sunset. (Wait until the following night for a clearer view with better contrast.) Simply take a digital photo of the moon on either the 27 or 28 and also on the night of apogee, nearly two weeks before on April 15. 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 near perigee moon sizes on a computer using the same image scale. The April 15 gibbous moon image will be found to have a diameter about 10% smaller than either the April 27 or April 28 images.

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 always presents the same hemisphere toward Earth as it orbits our planet. Therefore, we always see its same side; we never see the far side. Strangely enough, 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. Hence, the moon moves slowest at apogee and fastest at perigee. All the while, it rotates at a constant rate, completing one full rotation 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.

Observe with binoculars or a small telescope the dark-floored crater Grimaldi along the moon’s western edge (see image below). (If the moon is too bright, wear sunglasses.) Image it using a digital camera at full optical zoom. Be sure to focus the camera and be careful not to overexpose the image. Do this in the late evening on April 12, or 13, and again on April 20 or 21 in the pre-dawn sky. Download the two images on a computer displaying the same image scale. Closely examine the lunar surface from the western rim to Grimaldi and Hevelius. The April 21 image should show a greater distance to the lunar edge and reveal more craters than the April 12 image. Effectively, you are viewing slightly around the western edge on April 21.

LibrationGrimaldi page 001Click the image for a larger/print version

Share your images and observations with the world on Facebook, or Tweet using #GAM2017 hashtag (@gam_awb).