The Moon’s apparent size in our night sky changes, sometimes looking larger and other times smaller. This observed variation is not an optical illusion, but rather a real physical phenomenon determined by the Moon’s changing distance from Earth.
The Moon’s Orbit Around Earth
The Moon does not travel around Earth in a perfect circle. Instead, its path is an ellipse. This elliptical shape means that the distance between the Moon and Earth is not constant throughout its approximately 27.3-day orbit. The Earth is not positioned precisely at the center of this elliptical path, causing the Moon’s distance to vary as it completes each revolution. This fundamental characteristic of the Moon’s orbit is the primary reason for the changes in its apparent size.
Varying Distance: Perigee and Apogee
The elliptical nature of the Moon’s orbit leads to specific points where its distance from Earth reaches extremes. The point in the Moon’s orbit when it is closest to Earth is called perigee, while the point when it is farthest away is known as apogee. On average, the Moon’s distance at perigee is about 363,300 kilometers (approximately 225,700 miles), and at apogee, it is about 405,500 kilometers (approximately 251,970 miles). This difference of over 40,000 kilometers significantly impacts how large the Moon appears to us.
When the Moon is at perigee, its closer proximity makes it appear larger in the sky, much like a car looks bigger as it approaches. Conversely, when the Moon is at apogee, its greater distance causes it to appear smaller. The actual distances for perigee and apogee can vary slightly due to the gravitational influences of the Sun and other celestial bodies.
How Apparent Size is Measured
Astronomers quantify the apparent size of celestial objects using a measurement called angular diameter. This term describes how large an object appears from a specific viewpoint, expressed in degrees or arcminutes (where one degree equals 60 arcminutes). The Moon’s angular diameter varies depending on its distance from Earth.
At perigee, the Moon’s angular diameter can be around 33.9 arcminutes, while at apogee, it can shrink to approximately 29.7 arcminutes. This range illustrates the measurable difference in its apparent size. The concept of angular diameter helps scientists precisely track and predict these variations, providing a quantifiable way to understand the observed changes in the Moon’s visual dimensions.
Understanding Supermoons and Micromoons
The terms “Supermoon” and “Micromoon” are popular descriptions that directly relate to the Moon’s varying distance from Earth. A Supermoon occurs when a full moon nearly coincides with perigee, the point of its closest approach to Earth. This alignment makes the Moon appear noticeably larger and brighter than an average full moon. Supermoons can appear up to 14% larger and 30% brighter than the smallest full moon, which occurs at apogee.
Conversely, a Micromoon, also known as an Apogee Moon, happens when a full moon occurs near apogee, its farthest point from Earth. During a Micromoon, the Moon appears smaller and somewhat dimmer.
While the visual difference between a Supermoon and a Micromoon can be substantial when compared side-by-side, the change in apparent size during a single lunar cycle might be subtle for casual observers. These events are natural consequences of the Moon’s elliptical orbit, offering observable examples of how distance influences perceived size.