Projection Alarm Clocks

11 Comments

1. ble
   Posted August 14, 2010 at 1:23 pm | Permalink

   the clocks that are not atomic work on fm radio waves so as long as you can get fm radio in your house it should be no problem

2. dadick
   Posted September 30, 2010 at 2:44 pm | Permalink

   Well we have gps already, so if it had an atomic clock inside, maybe??

3. Mercury 2010
   Posted October 13, 2010 at 5:27 pm | Permalink

   There are a lot of answers here — some good, some not.

   One of the fundamental problems of timekeeping is that the earth's rotation is not uniform. In the long run, the earth gradually slows down; but there are larger short-term irregularities. For a long time, the second was defined as a certain fraction of the length of the year 1900 (based on the equinoxes). In 1967, scientists came up with a physical definition of the second (based on a certain transition of the element cesium) that matched, as well as possible, the definition based on 1900; and we have been using that definition ever since.

   Nominally, a day contains 86400 seconds (=24*60*60). The earth is now rotating a bit more slowly than it was in 1900. Even if the rotation remained fixed from now to the rest of eternity, the average number of seconds in the mean solar day would not be 86400, but about 86400.0004 (the current length of the mean solar day, in terms of the atomic definition of the second). Thus, we would have to add a leap second roughly every 2500 days (6.8 years) to keep Universal Time (which uses atomic seconds) in sync with the rotation of the earth. Adding this leap second periodically does not mean per se that the earth is slowing down; it simply means that the atomically-defined second is not exactly 1/86400 of a mean solar day.

   But the earth's rotation speed continues to change. In the long run it slows down, but this does not happen at a steady pace. The irregularity of this process means that leap seconds are not added at a regular interval, but rather whenever it is found observationally that the two systems of timekeeping (UT vs. the earth's rotation) have drifted apart by about a second.

   As the earth continues to slow down in the long term, we will have to add leap seconds more and more often, and that will make time calculations more difficult in the future. For this reason, some scientists have suggested that we replace the leap-second system with something else. (This is an ongoing debate at the present time.)

   Note that leap seconds have absolutely nothing to do with a leap year. Leap seconds come about because the atomic second doesn't currently match 1/86400 of the mean solar day, and because the earth's rotation continues to change irregularly (with a long-term slowing trend). Leap years come about because the number of days in a year is not an exactly integral number of days. Even if the earth's rotation were perfectly uniform, we would have the problem of leap years. The earth's changing rotation speed will, however, eventually lengthen the number of days in the year enough that we will have to modify the calendar system (but that won't happen for a long time).

   Now I'll discuss the common misconception people have. As I said, a leap second occurs for a combination of the following two reasons:
   1) The atomic second and the astronomically defined second do not match.
   2) The earth's rotation speed continues to change.

   Even if only #1 was true, we would have leap seconds. (And if #2 alone were true at first, then #1 would also soon be true.)

   Some people, however, incorrectly interpret a leap second to mean that the earth's daily rotation has changed by a full second. (In reality, the one-second discrepancy has accumulated over a few years.) Starting from this false premise, they jump to the conclusion that after we've added tens of thousands of leap seconds, it will mean that the earth's rotation will have stopped. This is a completely erroneous interpretation.

   In the long run, the length of the day is increasing very slowly — about 0.00025 seconds per century. You might think that this will bring the rotation almost to a stop eventually, but the rate will diminish over time. Eventually (after billions of years!), the earth and moon will become tidally locked. At that point, the sidereal rotation period of the earth will be fixed at about 47 current days (and this will also be the sidereal period of the moon's orbit). When that happens, not only will the same side of the moon always face the earth, but the same side of the earth will always face the moon.

   Hope that helps. Add some more notes in your question or email me if there's any confusion.

   – edit — comment on your note
   You're right that even if each leap second meant that the earth's rotation has slowed down by a second (and it doesn't mean that), it would take an infinitely long time for the rotation to stop. So whoever reaches that conclusion would be piling one misconception on top of another. I've never actually heard this combination of misconceptions, so I don't think I can explain the reasoning behind it.

   Let me just add one more thing to contrast leap seconds with leap years. The purpose of leap seconds is to make sure that the sun reaches its highest point at noon (on the average, after allowing for time zones, day-to-day variations related to the earth's elliptical orbit, etc.); in other words, it prevents the time of the sun's highest point from drifting in time from year to year. The purpose of leap years is to make sure that the vernal equinox always occurs on about the same day each year; that is, it prevents the vernal equinox (or any other equinox or solstice) from drifting through the calendar year. Leap seconds keep the time of day in sync with the sun, and leap years keep the calendar in sync with the seasons. (Nature didn't make it easy! It's our job to come up with a scheme to adapt to whatever nature has given us.)

   By the way, you can see that timekeeping is a messy subject, and there's quite a bit more to it than has been discussed here. Fortunately, most people in their everyday lives are not concerned with time to the nearest second, and most people don't adjust their watches when there's a leap second. (But today's "atomic" clocks and wristwatches do that automatically.)

4. raislak
   Posted October 16, 2010 at 8:50 am | Permalink

   Thanks for the tip. I go to Batteries Plus on Canal for all my rechargeable needs, but it never occurred to me to take it there. Duh
   Actually I went back today to have them fix or refund, but after several minutes of no one at the mini store, I walked around looking at solar watches with atomic clock setting (thinking spending $$ now will save me rebuying cheap watches every two years). But at a different mall mini shop (Dakota watches), I got waited on by someone I wish I had worked with yesterday, and I did get a new watch that didn't have all those expensive features.
   Now I got a dress up (with alarms) and work (no audio alarm) watch.

5. happyseacat
   Posted October 17, 2010 at 9:53 pm | Permalink

   And now an atomic clock is following me on Twitter. I swear to god, I cannot anticipate when this will happen.

6. sd_ducksoup
   Posted November 23, 2010 at 3:49 am | Permalink

   The temperature readings are from internal electronics. Most clocks like that will give internal temps; some have the added feature of external (usually wireless) sensors for outside temps.

   The "weather forecasts" are just based on the local barometric pressure and aren't particularly accurate. For instance, rising pressure = sunny, but that could also indicate a front moving through and some rain, or falling pressure = rain, but that could mean the front is passed and the skies are clearing.

   Umm — transmit? They are designed to receive the US atomic clock broadcasts on shortwave (5, 10, 15, and 20 MHz from Ft. Collins, CO or Hawaii) and interpret the signals to reprogram the clock to the correct time once a day (usually 1 to 4 AM local time).

   You may find that the one with weather also has a relative humidity sensor. If that would be useful where you live, then it might be a reason to choose it over the other.

7. sd_ducksoup
   Posted February 12, 2011 at 4:31 am | Permalink

   The temperature readings are from internal electronics. Most clocks like that will give internal temps; some have the added feature of external (usually wireless) sensors for outside temps.

   The "weather forecasts" are just based on the local barometric pressure and aren't particularly accurate. For instance, rising pressure = sunny, but that could also indicate a front moving through and some rain, or falling pressure = rain, but that could mean the front is passed and the skies are clearing.

   Umm — transmit? They are designed to receive the US atomic clock broadcasts on shortwave (5, 10, 15, and 20 MHz from Ft. Collins, CO or Hawaii) and interpret the signals to reprogram the clock to the correct time once a day (usually 1 to 4 AM local time).

   You may find that the one with weather also has a relative humidity sensor. If that would be useful where you live, then it might be a reason to choose it over the other.

8. TheRealOBR
   Posted February 18, 2011 at 10:23 pm | Permalink

   Just posted: Travel Alarm Clock: Reviews of the Best Digital and Atomic Clocks: When I travel I like to have a r…

9. drdaddys
   Posted February 27, 2011 at 1:43 am | Permalink

   wrong…23:59:60 is the extra second

10. Mercury 2010
    Posted December 11, 2011 at 8:43 pm | Permalink

    The definition of a "second" was set so that there are 86,400 seconds in a day — back around the year 1840! That is the average date for the observations that were used to define the second (i.e., about 1790 to 1880's). Since then, then Earth has slowed down a tiny bit for a while, and sped up for a while. Since 1972, there have been 23 leap seconds, over a period of 13,149 days. That means that ON AVERAGE, over the last 36 years, the days have been longer than 86,400 seconds by 0.00175 seconds. That extra part is called the "excess length of day", and adds up to about 0.6 or 0.7 seconds per year.

    Now, we might suppose that the Earth could continue at that rate forever, with each day being 86,400.00175 seconds (average over several decades). The would be zero deceleration. But in fact, the Earth is decelerating. In about 100 years, we can expect that the day will be 86,400.00375 seconds long. Then leap seconds will have to put in about every 267 days (average). And in 1000 years from now, the day will be 86,400.02175 seconds long, and leap seconds will be needed every 46 days (as always, on average). (The detailed situation is a bit more complicated.) Some people are worried about this — see the second link below.

    There is a graph of the "excess" length of day at the first link. Look for the graph labeled "BullA LODS".

    For a more technical paper, try the third link.

11. Brister4296disc
    Posted January 20, 2012 at 7:52 pm | Permalink

    Chaney Instruments Acu-Rite 75077 Wireless Weather Forecaster with Remote Sensor and Atomic Clock Reviews: