See the big moon illusion for yourself on the evening of April 5th or 6th. 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.
- 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 for two hours or more and repeat the observation. (If it is a morning observation, first look a couple of hours before sunrise.)
- 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. Perigee occurs on April 16, 2023, when the moon shows a crescent, and is low in the east before sunrise. Apogee falls on April 28, 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 15th or 16, shortly before sunrise. Take another image in the evening of apogee, April 28, 2023. 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 perigee and apogee moon sizes on a computer using the same image scale. The April 16 image will be found to be about 8% larger than the April 28 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 the crater Grimaldi near the moon’s western edge. It has a dark floor, making for easy identification. Photograph 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 on April 7 or 8, and do so again before sunrise on April 15. Download the images on a computer displaying the same image scale. Closely examine the amount of lunar surface between the western edge to Grimaldi. The April 15 image should show much less distance than the April 7-8 image.
See the accompanying Libration Grimaldi 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.)
Spotting Mercury
a naked eye or binocular activity
The first two weeks of April provide a good time to spot Mercury, the planet closest to the sun. Because it is always close to the sun, it never appears very high above the horizon either after sunset or before sunrise. But in early April, the conditions are right to see this little world shining in the bright evening twilight.
Look low in the west 40 minutes after sunset between April 3 and 15. The best evenings likely will be April 7-10. Mercury will lie just above the horizon, far below the eye-catching Venus. Before April 3 and after April 15, it can be a challenge to spot the little planet in the bright twilight sky.
See “Spotting Mercury” Graphic.
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.
- 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.
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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 are able to easily able to see crisp edges of the lunar maria and can see the changing effects of lunar libration.
April is a month for brilliant Venus
a naked eye and binocular activity
While easy to spot, the planet currently lies too far from Earth to see its phase when viewed through binoculars. A small telescope, however, will reveal a small gibbous shape.
- At the beginning of April, Venus lies above the western horizon sixty minutes after sunset.
- On April 13 and 14, it lies between the Pleiades and Hyades star clusters.
- On April 23, it lies east of the crescent moon.
- On April 24, it lies west of the crescent moon.
Mars passes Mebsuta, a moderately bright star in Gemini
a naked eye, binocular, and telescope activity
- Throughout April, Mars moves across the constellation of Gemini. It will be high in the west-southwest 90 minutes after sunset.
- On April 14 at 18:00 UT, Mars lies just 9 minutes south of the moderately bright star Mebsuta. From a few hours before that time to a few hours after, use binoculars to watch the planet slowly slide below the star.
See Mars and Mebsuta diagram.
