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Perigee and Apogee

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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)

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Comments

When rockets head for the

By Ken Gibson

When rockets head for the ISS,

why is Berthing and Docking used, are they not the same?

So it seems logical that

By Ejazz

So it seems logical that summers in the Southern hemisphere, when earth is at perigee and closest to the sun, should be a bit hotter and sunlight more intense, than summers on this side of the equator? What think?

When the sun is farthest away

By Deathride

When the sun is farthest away from the earth in the summer, why is it the warmest weather?
Seems that it would be the coolest weather.

It would seem to make sense,

By Almanac Staff

It would seem to make sense, wouldn't it? But actually the hotter/cooler weather, and seasons, are caused by the angle of the incoming sunlight. The more direct an angle, the more heat we will receive. Earth is tilted at a 23.5 degree angle to the plane of its orbit; this tilt can be toward the Sun (summer in Northern Hemisphere), away from the Sun (winter in the Northern Hemisphere), or neither (spring and fall equinoxes). Because of this tilt, at any one time, the amount and angle of sunlight will differ in various locations on the planet. And this distribution will change as Earth travels along in its orbit.

From January's perihelion (when Earth is closest to the Sun), we move farther away from the Sun each day. In January, the angle of the Sun’s rays were low in the Northern Hemisphere, so the Sun’s energy was distributed over a larger area, weakening its intensity in any one place (sort of like the light from a tilted flashlight on a surface); the days were also shorter than they are now, so that we received less energy from the Sun.

At the spring and fall equinoxes, we are neither tilted toward nor away from the Sun, which means that at the Equator, the Sun’s rays are coming in at about a 90 degree angle—so the energy there is concentrated along a smaller surface. However, at the higher latitudes, the Sun’s rays are being received at more of an angle, so we’re getting less intense rays than the Equator, and things are a bit cool. As the spring days progress toward the summer solstice, the angle of the Sun’s rays become more direct, and the days are also getting longer, which increases the amount of energy that we are receiving. So, we’re getting more energy as spring progresses to summer, resulting in warmer temperatures. Hottest temperatures are usually a bit after the summer solstice, because it takes a while for the water and land to reach a certain rate of absorbing vs. releasing the Sun's energy.

Thank you for the info. My

By Tammantha

Thank you for the info. My favorite part is "orbit not to scale" which made me laugh. Thanks!

Very useful info!

By Joe Robinette

Very useful info!

Hi, Can you please if

By panda_snot

Hi,
Can you please if available post the Perigee and Apogee days for all 2014?
Also interested how Perigee and Apogee affects sensitive people.

Thanks!
Daniel
Texas

You can find the times of

By Almanac Staff

You can find the times of perigee and apogee for the Moon on the appropriate dates of the right-hand pages of The Old Farmer's Almanac, both print and digital versions. These times/dates have been converted to Eastern Time (National, Canadian, Southern editions) or Pacific Time (Western edition).

For a list of perigee and apogee dates online, you might go to the following; please keep in mind that these dates are listed in Universal Time (at Greenwich, England) and would need to be converted to your local time zone.

http://fourmilab.ch/earthview/pacalc.html

We have not found information about how perigee and apogee might affect people, at least nothing conclusive, but good luck with your research!

I love learning about this

By Petlover1

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

By Almanac Staff

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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