{"id":123,"date":"2020-02-03T20:18:26","date_gmt":"2020-02-03T20:18:26","guid":{"rendered":"https:\/\/pixsilver.com\/Sky_School\/?page_id=123"},"modified":"2020-02-03T20:18:26","modified_gmt":"2020-02-03T20:18:26","slug":"zero-hour","status":"publish","type":"page","link":"https:\/\/pixsilver.com\/Sky_School\/zero-hour\/","title":{"rendered":"Zero Hour"},"content":{"rendered":"\n

This time we take a fishing trip as we continue our tour of the faint and obscure constellations. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Pisces is well known, as it\u2019s one of the Zodiac constellations. These\n constellations had (and unfortunately are still given) astrological \nsignificance; they lie on the imaginary line of the Ecliptic. To \nastronomers ancient and modern, the Ecliptic is the path followed by the\n planets and the Sun and, more loosely, our Moon. Since Copernicus, \nwe\u2019ve known it\u2019s the plane of Earth\u2019s orbit projected out into space. \nNot surprisingly, it cuts the celestial equator at an angle of 23.5 \ndegrees \u2013 the same as Earth\u2019s tilt. The ecliptic and the celestial \nequator currently meet in Pisces at a point known, oddly enough, as \u201cthe\n First Point in Aries.\u201d <\/p>\n\n\n\n

The dashed line marked 0h on the diagram is the celestial Prime \nMeridian \u2013 the Zero-hour line. This line joins the poles with the First \nPoint in Aries. It does the same job as Earth\u2019s zero meridian, the one \nthat goes through Greenwich, England. The \u201clongitudes\u201d of objects in the\n sky are measures East and West of this line as hours and minutes, just \nas earthly longitudes are measured east and west of Earth\u2019s prime \nmeridian. The nearer point where the ecliptic crosses Pisces marks One \nHour (1h), while the sharp angle in the constellation lies on 2h. <\/p>\n\n\n\n

When the Sun, in its annual wander along the ecliptic, crosses the \nequator coming north, our Spring begins. The day and night are equal \nlength, and we say we are at the Spring (or Vernal) Equinox (equi-equal \nnox-night). This happens when the Sun passes through the First Point in \nAries. You can probably guess now why it\u2019s called that \u2013 it has to do \nwith the constellations\u2019 slow drift across the sky caused by earth\u2019s \nprecession. Thousands of years ago, the First Point in Aries was, in \nfact, in Aries. <\/p>\n\n\n\n

To see this in your astronomy software, turn on the celestial grid \nlines, the ecliptic and the constellation lines. If you can, turn on a \nlabel for the vernal equinox, too. Set the date and time for noon on \nMarch 21; the Sun should be on the Vernal Equinox. Now switch the AD to \nBC and watch the equinox and the Sun jump back into Aries. You might \nhave to adjust the date by a couple of weeks to get everything aligned, \nthough. <\/p>\n\n\n\n

Deep Sky Effect<\/strong>
As I pointed out above, the \nzero meridian is defined by the line through the poles and the first \npoint in Aries, and that point is moving. One side effect of this is \nseen in the arrangement of the NGC deep-space objects. They were \nnumbered rationally, starting near the north celestial pole at the zero \nhour meridian. From there the numbers increase toward the south pole, \nthen move a bit west and so on. By the time the whole sky has been \ntraversed, back around to the 23h59m meridian, you\u2019d be at the \nlast-numbered NGC object. However, as you\u2019ll see in any good star chart,\n it\u2019s not quite like that; the precession of the equinoxes (i.e. earth\u2019s\n wobble) has moved the zero meridian enough since the creation of the \nNew General Catalogue (for that is what NGC stands for) that the \nhighest-numbered NGCs are now to the east of the line. The precession is\n also why sky atlases have to be updated a couple of times a century. <\/p>\n\n\n\n

\n\n\n\n

Related Book Review<\/strong>
For a wonderful account of \nthe competition to find a way to measure longitude, read Dava Sobel\u2019s \n\u201cLongitude\u201d. It\u2019s a tale of a huge prize, scientific rivalry, bias and \ngreed. Even Galileo got in on it, thinking that the moons of Jupiter, \nwhich he had just discovered, offered a way to measure longitude. They \ndidn\u2019t, but years of measurements of the moons\u2019 orbits gave the first \ndirect evidence that the speed of light is finite. It\u2019s a fascinating \nbook, and is probably in your local library. <\/p>\n","protected":false},"excerpt":{"rendered":"

This time we take a fishing trip as we continue our tour of the faint and obscure constellations. Pisces is well known, as it\u2019s one of the Zodiac constellations. These constellations had (and unfortunately are still given) astrological significance; they lie on the imaginary line of the Ecliptic. To astronomers ancient and modern, the Ecliptic […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"templates\/template-full-width.php","meta":{"footnotes":""},"class_list":["post-123","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/pages\/123","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/comments?post=123"}],"version-history":[{"count":1,"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/pages\/123\/revisions"}],"predecessor-version":[{"id":124,"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/pages\/123\/revisions\/124"}],"wp:attachment":[{"href":"https:\/\/pixsilver.com\/Sky_School\/wp-json\/wp\/v2\/media?parent=123"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}