{"id":10663,"date":"2016-12-30T22:58:27","date_gmt":"2016-12-30T14:58:27","guid":{"rendered":"http:\/\/stuif.com\/blog\/?p=10663"},"modified":"2016-12-30T22:58:27","modified_gmt":"2016-12-30T14:58:27","slug":"wait-a-second","status":"publish","type":"post","link":"https:\/\/stuif.com\/blog\/?p=10663","title":{"rendered":"Wait a second!"},"content":{"rendered":"

Tomorrow, 31 December 2016, just before midnight, an\u00a0extra\u00a0second will be added to the Universal Coordinated Time \u00a0(UTC)!\u00a0It is called a leap second<\/span>.<\/p>\n

Why??<\/span><\/p>\n

Probably everybody will be familiar with the concept of a leap day\u00a0. A year in the international \u00a0calendar has 365 days, but the solar year is a bit longer, 365.25 days. To keep this calendar synchronised with the solar year, \u00a0every four years an extra day (29 February) is added to the calendar, a leap day<\/span>. 2016 was a leap year, the next one will be 2020.<\/p>\n

The Chinese calendar is based on the motion of the Moon, orbiting the Earth with a period of 29.53 days. A (lunar) year is 12 months =\u00a012 \u00d7 29.53 = 354.36 days, about 11 days shorter than the solar year. To keep this calendar synchronised with te solar year, every two\/three years an extra month is added, a leap month<\/span>. Next year will be a leap year in the Chinese calendar, it will have 13 months\u00a0with one of the months duplicated. Not always the same month, this time the 6th month. More detailed information about calendars can be found on my website<\/a><\/p>\n

For those not familiar with UTC, it is\u00a0the primary time standard\u00a0by which the world regulates clocks and time. It is basically the solar time at 0\u00b0 longitude, with the solar day as fundamental unit. The 0\u00b0 meridian passes through Greenwich, therefore UTC is sometimes called Greenwich Mean Time (GMT). The world has been divided into 24 time zones, they\u00a0are defined as UTC plus or minus a number of hours. For example Malaysian time is UTC + 8.<\/p>\n

So, the UTC is based on the (solar) day and a day is 24 x 60 x 60 = 86400 seconds, right? Why do we need to add a leap second?\u00a0The answer is\u00a0simple, but may surprise you.<\/p>\n

A (solar) day is not exactly 86400 seconds!<\/span><\/p>\n

Here is a graph of the “extra” length of day over the last few decades. Click to enlarge and see more details<\/p>\n

\"deviation-of-day-length-from-si-day\"<\/a><\/p>\n

It is only a few milliseconds every day, but it accumulates! Therefore it has been decided, in 1972, to add an extra second to UTC, when this accumulated deviation gets more than 0.9 second. The red graph shows when leap seconds were inserted. \u00a0As you see, the deviation from 85440 seconds is quite irregular and actually not predictable. That’s why the leap seconds are announced only 6 months in advance.<\/p>\n

Why are the deviations always positive? That has an interesting, physical, reason. It is because of the moon! The moon is responsible for\u00a0the tides, causing friction! This friction slows down the rotation of the Earth! It is a small but real effect,\u00a0the solar day increases about\u00a01.4\u20131.7 milliseconds\u00a0per century. There is geological evidence that about 500 million year ago, the length of the day was shorter, ~ 22 hours.<\/p>\n

These tidal\u00a0forces<\/a> and\u00a0eventually tidal locking<\/a>\u00a0might be\u00a0an interesting. topic for a separate blog.<\/p>\n

A few remarks<\/p>\n

    \n
  1. The leap second will be added to\u00a0UTC, 31\u00a0December at midnight. 23:59.59 will not be followed by 00.00.00 but first by 23.59.60<\/li>\n
  2. In Malaysia (UTC + 8) the leap second will be added on 1 January. \u00a007:59:59 should not be followed by 08:00:00 but first by 07:59:60.<\/li>\n
  3. Computer guys are not happy with an insertion\u00a0of an extra second. It may cause computer\u00a0failure. The Google engineers\u00a0will just slow down the system clock slightly, from 10 hours before, until 10 hours after midnight, resulting in 1 second extra…:-) \u00a0Technical details here<\/a><\/li>\n
  4. Time reckoning is a complicated topic. I have simplified it here…:-)<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"

    Tomorrow, 31 December 2016, just before midnight, an\u00a0extra\u00a0second will be added to the Universal Coordinated Time \u00a0(UTC)!\u00a0It is called a leap second. Why?? Probably everybody will be familiar with the concept of a leap day\u00a0. A year in the international … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false},"version":2}},"categories":[17,7],"tags":[],"class_list":["post-10663","post","type-post","status-publish","format-standard","hentry","category-physics","category-science"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p2LqIR-2LZ","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10663","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10663"}],"version-history":[{"count":22,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10663\/revisions"}],"predecessor-version":[{"id":10724,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/10663\/revisions\/10724"}],"wp:attachment":[{"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10663"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10663"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stuif.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10663"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}