What is a Supermoon?
A Supermoon is a popular term for when a full or new moon coincides with the moon's closest approach to Earth in its elliptical orbit. This closest point is called perigee. When the moon is at perigee during its full phase, it appears visibly larger and brighter from Earth compared to an average full moon.
Key Principles: Perigee and Full Moon
The moon's orbit around Earth is not a perfect circle but an ellipse, meaning its distance from Earth varies throughout the month. Perigee is the point in this orbit when the moon is closest to Earth, while apogee is the farthest point. A Supermoon occurs when a full moon (the phase where the entire side facing Earth is illuminated by the sun) happens within approximately 90% of perigee.
A Practical Example
Imagine observing the full moon on two different nights. On one night, the moon is at its apogee (farthest point), appearing smaller. On another night, it's at perigee (closest point) and is also a full moon. This latter instance is a Supermoon, and the moon will look about 14% larger in diameter and 30% brighter than the apogee full moon, though this difference can be subtle to the naked eye without direct comparison.
Importance and Perception
While the visual difference in size and brightness might not be immediately obvious without comparison, Supermoons are significant astronomical events that draw public attention to lunar cycles and orbital mechanics. They can also lead to slightly higher tides due to the moon's increased gravitational pull at perigee, though this effect is usually minor and not a cause for alarm.