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EmailThe 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).
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)
Comments
I love learning about this
I love learning about this topic. Can info on day length and how it varies at different latitudes be found or gray lines and what exactly does the time a new season starts indicate?
Dear Petlover 1, For more
Dear Petlover 1,
For more information about what it means for a season to begin, as far as the astronomical definition, please see:
http://www.almanac.com/content/first-day-seasons
As for the gray lines, also called the terminator (the cut-off point between night and day on a planet or moon), you can see this when you look at the first and last quarter phases of the Moon--the border between the light and dark side of the Moon. As for Earth, a handy site is the following, which allows you to see the terminator for a range of times and years. You do need to input the desired time in Universal Time:
http://aa.usno.navy.mil/data/docs/earthview.php
And what Earth looks like currently (simulated view)
http://aa.usno.navy.mil/imagery/earth/live
You can check day length for any location at the following Almanac.com page. Just input the desired location and date:
http://www.almanac.com/astronomy/rise/NH/Peterborough/2012-06-7
If you wish a table of day length information for an entire year for a particular location, you can purchase the digital version of the Left-Hand Calendar Pages, called MyLocalAlmanac, which calculates astronomical data for your area.
http://www.almanac.com/product/my-local-almanac
Keep in mind, length of day mentioned in the Almanac is the time between sunrise and sunset. Sunrise starts when the upper edge of the sun appears on the horizon. Sunset begins when the upper edge of the sun disappears below the horizon.
Because of refraction and other optical effects in the atmosphere, light is still present when the Sun is close to the horizon but not visible. This is called twilight. Twilight length, as well as day length, is dependent on latitude. There is a page on twilight in the Almanac, for more information about calculating length and the different types of twilight.
Hope this helps!
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