Ever heard the terms perigee and apogee? Here is a quick definition.
The Moon's orbit periodically grows rounder and then more oval; it is never a perfect circle.
- The point at which it is nearest Earth each month is called its perigee (this varies throughout the year).
- The point at which the Moon is farthest from Earth each month is called its apogee (this varies throughout the year as well).
In recent times, the news media has taken to calling a full Moon near perigee a "Supermoon" because its nearness to Earth causes the Moon to appear bigger and brighter. This effect can be enhanced by what we call the "Moon Illusion ."
Image credit: NASA
Perigee and apogee also affect the tides.
During the final quarter of the 20th century, the Moon was as close to Earth as 216,500 miles (surface to surface) at its most extreme perigee and as far away as 247,700 miles at its most extreme apogee. At perigee, tidal ranges are increased; at apogee, they are decreased.
To a lesser degree, the Sun has a similar tidal effect, a consequence of its location just 91.5 million miles away in early January but some 94 million miles distant in July.
The punctual tide draws up the bay
With ripple of wave and hiss of spray.
–Susan Coolidge, American writer (1835-1905)