An Extra Second

Today will be 86,401 seconds long. That number may seem odd, particularly when you consider that most days are 86,400 seconds long. The difference is a result of the leap second being added to our clocks at the end of June 30, 2015 at 23:59:59.

Observatory Clocks
Two new GPS time servers installed in the Keck 2 computer room
The need for an extra second comes from the irregular rotation of our planet. Due to gravitational interaction with the Moon our planet is gradually slowing its rotation. Other shifts can also cause slight irregularities in the planetary rotational rate. In order to keep Coordinated Universal Time in phase with the Earth’s rotation it is necessary to insert an extra second every so often when the error becomes too large.

Since zero hours universal time occurs at 14:00 Hawaii Standard Time, this extra second will occur at 2pm in the afternoon for my readers in the islands. If you are watching a clock that can account for this leap second you will see this second appear. The clock can do a few things, it could simply stop for a second, holding at 23:59:59 for two seconds. The clock could also count the extra second, displaying 23:59:60.

Update: A small group of true nerds gathered in the Keck 2 computer room at 0hUT today and watched our timeserver do leap second. The display notably remained at 23:59:59 for two seconds.

Generally only high precision time servers and GPS receivers will contain software to correctly implement the leap second. Most clocks do not correct for leap seconds, it is just not that important that this feature be implemented perfectly in most devices.

Network connected clocks will simply adjust the time on the next opportunity. Connected clocks routinely synchronize with an online time standard. Your computer requests the correct time via NTP protocol from an internet time server once each day, this is the usual default configuration for a net connected computer unless you have changed it. Cell phones get their time from the network and may receive their extra second more quickly, maybe within a few moments of the leap.

The rest of the clocks in your life are usually more than a few seconds off in any case.

Last time this occurred in 2012 there were some troubling software crashes around the world. Airline departures were delayed and some major websites went down. There is some concern that similar occurrences may occur this time. As a result some experts have argued to do away with leap seconds and just let the error accumulate. The argument rages and we may see leap-seconds abolished, in the meantime this one will occur.

Now that you know about it, enjoy your extra second today. Remember to watch that clock at zero hours universal time and see what happens.

A Matter of Time

Time… It is simply a matter of time. At 00:00UT May 3rd, many of the observatory computers suddenly started reporting that the date was September 17th, 1995. To say that this created some problems is a dramatic understatement.

Datum TS2100 Time Servers
Two Datum TS2100 time servers installed above the telescope drive control computer (TDC)

The problem came from the primary observatory clock. This clock, properly called a time server, uses GPS signals to create a time reference that is accurate to microseconds. This is made possible by referencing to the atomic clocks carried by each GPS satellite. A time server is intricately connected to the network to distribute this time. Any computer in the building can ask it for time via the NTP protocol, but that has some inaccuracy due to network delays. For equipment requiring more precise time the server distributes a hard wired time reference using the IRIG-B protocol or a 1PPS timing pulse.

Without accurate time a telescope will simply not point in the correct direction. The calculation that the computers perform must take into account our rotating planet. Feed incorrect time to that calculation and you will point to the wrong piece of sky. A few milliseconds off can result in a pointing error of arcseconds, a large error for a large telescope.

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