{"id":2600,"date":"2012-01-24T12:23:16","date_gmt":"2012-01-24T17:23:16","guid":{"rendered":"http:\/\/spinor.info\/weblog\/?p=2600"},"modified":"2013-03-10T09:14:59","modified_gmt":"2013-03-10T13:14:59","slug":"discontinuous-time","status":"publish","type":"post","link":"https:\/\/spinor.info\/weblog\/?p=2600","title":{"rendered":"Discontinuous time"},"content":{"rendered":"<p>When I write about things like <a href=\"https:\/\/www.vttoth.com\/CMS\/projects\/242\">precision orbit determination<\/a>, I often have to discuss the difference between <a href=\"http:\/\/en.wikipedia.org\/wiki\/Ephemeris_time\">ephemeris time<\/a> (ET) and <a href=\"http:\/\/en.wikipedia.org\/wiki\/Coordinated_Universal_Time\">coordinated universal time<\/a> (UTC). ET is a &#8220;clean&#8221; time scale: it is essentially the time coordinate of an inertial coordinate frame that is attached to the barycenter of the solar system. On the other hand, UTC is &#8220;messy&#8221;: it is the time kept by noninertial clocks sitting here on the surface of the Earth. But the fact that terrestrial clocks sit inside the Earth&#8217;s gravity well and are subject to acceleration is only part of the picture. There are also those blasted leap seconds. It is because of leap seconds that <a href=\"http:\/\/en.wikipedia.org\/wiki\/International_Atomic_Time\">terrestrial atomic time<\/a> (TAI) and UTC differ.<\/p>\n<p>Leap seconds arise because we insist on using an inherently wobbly planet as our time standard. The Earth wobbles, sometimes unpredictably (for instance, after a major earthquake) and we mess with our clocks. Quite pointlessly, as a matter of fact. And now, we missed another chance to get rid of this abomination: the International Telecommunication Union <a href=\"http:\/\/www.nature.com\/news\/leap-second-granted-extra-time-1.9865?WT.ec_id=NEWS-20120124\">failed to achieve consensus<\/a>, and any decision is postponed until 2015.<\/p>\n<p>For the curious, an approximate formula to convert between TAI and ET is given by\u00a0ET \u2013 TAI = 32.184 + 1.657\u00d710<sup>\u20133<\/sup> sin <em>E<\/em>, where\u00a0<em>E<\/em> = <em>M<\/em> + 0.01671 sin <em>M<\/em>, <em>M<\/em> = 6.239996 + 1.99096871\u00d710<sup>\u20137<\/sup> <em>t<\/em> and <em>t<\/em> is the time in seconds since <a href=\"http:\/\/en.wikipedia.org\/wiki\/J2000.0#Julian_years_and_J2000\">J2000<\/a> (that is, noon, January 1, 2000, TAI). To convert TAI to UTC, additional leap seconds must be added: 10 seconds for all dates prior to 1972, and then additional leap seconds depending on the date. Most inelegant.<\/p>\n<p>Speaking of leap this and that, I think it&#8217;s also high time to get rid of daylight savings time. Its benefits are <a href=\"http:\/\/en.wikipedia.org\/wiki\/Daylight_saving_time#Dispute_over_benefits_and_drawbacks\">dubious at best<\/a>, and I find the practice unnecessarily disruptive.<\/p>\n<fb:like href='https:\/\/spinor.info\/weblog\/?p=2600' send='true' layout='standard' show_faces='true' width='450' height='65' action='like' colorscheme='light' font='lucida grande'><\/fb:like>","protected":false},"excerpt":{"rendered":"<p>When I write about things like precision orbit determination, I often have to discuss the difference between ephemeris time (ET) and coordinated universal time (UTC). ET is a &#8220;clean&#8221; time scale: it is essentially the time coordinate of an inertial coordinate frame that is attached to the barycenter of the solar system. On the other <a href='https:\/\/spinor.info\/weblog\/?p=2600' class='excerpt-more'>[&#8230;]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20,3,18,26],"tags":[],"class_list":["post-2600","post","type-post","status-publish","format-standard","hentry","category-astronomy","category-physics","category-society","category-space","category-20-id","category-3-id","category-18-id","category-26-id","post-seq-1","post-parity-odd","meta-position-corners","fix"],"_links":{"self":[{"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/posts\/2600","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2600"}],"version-history":[{"count":8,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/posts\/2600\/revisions"}],"predecessor-version":[{"id":4746,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=\/wp\/v2\/posts\/2600\/revisions\/4746"}],"wp:attachment":[{"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2600"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2600"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/spinor.info\/weblog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}