{"id":632,"date":"2012-04-22T00:00:08","date_gmt":"2012-04-22T00:00:08","guid":{"rendered":"http:\/\/www.skycaramba.com\/blog\/?p=632"},"modified":"2012-04-21T14:36:20","modified_gmt":"2012-04-21T14:36:20","slug":"the-moon-goes-to-extremes","status":"publish","type":"post","link":"https:\/\/www.skycaramba.com\/blog\/2012\/04\/22\/the-moon-goes-to-extremes\/","title":{"rendered":"The moon goes to extremes"},"content":{"rendered":"

SkyCaramba weekly astronomy blog for the week ending April 28, 2012<\/em><\/p>\n

During the next few weeks, the moon will go from being its farthest from us all year, to its closest, to its farthest again.\u00a0<\/p>\n

The moon\u2019s orbit isn\u2019t a perfect circle. A few artificial satellites have perfectly circular orbits, but none of the natural ones discovered so far do. During their courses around their parent bodies, natural satellites get closer and then farther away.\u00a0<\/p>\n

The farthest point in the moon\u2019s orbit around the earth is called apogee. The closest point is called perigee. The gee<\/em> part comes from geo<\/em>, a root word for Earth. There\u2019s an apogee and a perigee every orbit. The distance from the center of the earth to the center of the moon changes by around 50,000 km from apogee to perigee or vice-versa.\u00a0<\/p>\n

The earth is also in a slightly non-circular orbit around the sun. The root word for sun being helios<\/em> in the astronomer\u2019s word system, the earth is farthest from the sun at aphelion and closest to it at perihelion. The fact that the sun, moon, and earth all exert gravitational forces on each other means the earth and moon aren\u2019t orbiting in perfect ellipses either.\u00a0<\/p>\n

Earth has more mass than the moon. If you could weigh Earth and the moon in the same gravitational field, Earth would weigh more. And something with more mass exerts more gravitational attraction. So while the moon\u2019s gravity tugs on the earth, Earth\u2019s gravity tugs more on the moon. Because of the moon\u2019s pull on the earth, Earth\u2019s path around the sun is actually a wiggly ellipse.\u00a0<\/p>\n

And while Earth makes its wiggly elliptical trek around the sun, it gets tugged a little more here or a little less there. Earth\u2019s speed changes slightly throughout this do-se-do. So as the moon goes from apogee to apogee, it\u2019s a little closer to Earth at some apogees than others. This month\u2019s and next month\u2019s apogees are the farthest of the year. And the perigee of May is the closest.\u00a0<\/p>\n

\"The<\/a>On April 22, 2012 at 14:03 UT, the moon will be 406,500 km from Earth. It\u2019ll be the same distance again on May 19 at 16:28. All other apogees this year are a little closer than that. The moon\u2019s closest perigee is on May 6 at 03:40 when it will be 356,900 km away.\u00a0<\/p>\n

So how much bigger or smaller will the moon look at these extremes? It\u2019s hard for us to notice the difference when the moon\u2019s phase is different and it\u2019s been two weeks. But photography enables us to make side by side comparisons. When pictures are taken with the same equipment at the same settings and with the moon at the same phase, we can see how the moon does look slightly smaller at apogee.\u00a0<\/p>\n

For the April 22 apogee, the moon\u2019s disk would take up a little less than 1,800 seconds of arc in our sky. It\u2019ll be a very thin crescent just after new moon, so you won\u2019t have much opportunity to see it. For the May 6 perigee, which happens to be a full moon, it takes up a little more than 2,000 second of arc. The moon\u2019s disk will look 11% bigger.\u00a0<\/p>\n

\u00a1SkyCaramba!\u00a0<\/p>\n

Here are a few web sites that explain the above topic more.\u00a0<\/p>\n

http:\/\/blogs.discovermagazine.com\/badastronomy\/2008\/09\/29\/the-moon-that-went-up-a-hill-but-came-down-a-planet\/<\/a>\u00a0<\/p>\n

http:\/\/blog.world-mysteries.com\/science\/ancient-timekeepers-part4-calendars\/<\/a>\u00a0<\/p>\n

http:\/\/www.fourmilab.ch\/earthview\/moon_ap_per.html<\/a>\u00a0<\/p>\n

http:\/\/www.fourmilab.ch\/earthview\/pacalc.html<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

During the next few weeks, the moon will go from being its farthest from us all year, to its closest, to its farthest again.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[150,276,29,142,90,275,152,166],"_links":{"self":[{"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/posts\/632"}],"collection":[{"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/comments?post=632"}],"version-history":[{"count":3,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/posts\/632\/revisions"}],"predecessor-version":[{"id":636,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/posts\/632\/revisions\/636"}],"wp:attachment":[{"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/media?parent=632"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/categories?post=632"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.skycaramba.com\/blog\/wp-json\/wp\/v2\/tags?post=632"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}