The Moon often appears notably larger and brighter in September because it frequently coincides with a celestial event known as a supermoon. This phenomenon creates the illusion of a significantly oversized lunar disk in the night sky.
Understanding the Supermoon Phenomenon
A supermoon is not a rare occurrence, but it creates a captivating visual display. For the Moon to be considered a supermoon, two specific astronomical conditions must align:
- Perigee Alignment: The Moon must be at its perigee, which is the point in its elliptical orbit when it is closest to Earth. The Moon completes its orbit around Earth approximately every 27 days.
- Full Phase Coincidence: Simultaneously, the Moon must be in its full phase. A full moon occurs about every 29.5 days, when the entire face of the Moon is illuminated by the Sun as seen from Earth.
When these two events—perigee and full phase—happen at roughly the same time, the result is a supermoon. Because the Moon is physically closer to Earth, it appears larger and more luminous than an average full moon.
Why September's Moon Can Appear Larger
While supermoons can occur at other times of the year, September's full moon is often highlighted as a supermoon. This means that during this month, the Moon's 27-day orbital cycle brings it to its closest point to Earth, precisely when it is also undergoing its 29.5-day cycle to reach the full phase. This perfect alignment results in a visually impressive sight, making the Moon seem exceptionally big and bright to observers.
Key Characteristics of a September Supermoon:
- Increased Apparent Size: Due to its closer proximity, the Moon can appear up to 14% larger than when it's at its furthest point from Earth (apogee).
- Enhanced Brightness: It can also seem up to 30% brighter, making it a prominent feature in the night sky.
- Optimal Viewing: Clear September evenings often provide excellent opportunities to witness this enhanced lunar display.
While the "largeness" is primarily due to its closer proximity, atmospheric conditions and optical illusions (like the Moon illusion, where the Moon appears larger near the horizon) can further contribute to the perception of an exceptionally big Moon.
