Post by Eli the Bearded Post by David Kaye
I could never figure out why the USA has two time-keeping organizations.
Must be a turf war or something. I've noticed that they diplomatically
Casual time keeping is very straight forward. Accurate time keeping is
very complicated. What is the "real" time?
Here are several different "times" that can be correct anywhere on Earth.
1) Local solar time (defined as noon coincides with sun's zenith for
that longitude), in apparent hours.[*]
2) Local solar time (defined as noon coincides with sun's zenith for
that longitude), in mean hours.[*]
3) Local time (UTC with local time zone applied). This one will get you
to your dentist on time. UTC is, roughly, definition 2 for Greenwich.
4) Straight out UTC. This one you want for your automatic logging.
5) Sidereal time and later UT1, which is measured against stars (then
quasars) in the night sky, and is the basis for adjusting for leap
seconds. This is the one used to measure the accuracy of all the rest.
UTC is kept within 0.9 SI seconds of UT1.
[*] Apparent hours vary with season ("equation of time"). Mean hours
are standard length. Solar apparent time is the one you will use
on Gilligan's Island. Solar mean time with your sextant and
[**] Dava Sobel's _Longitude_ (2007) book is well-known and covers how
accurate time keeping was important for sextant use. But David
Landes' _Revolution in Time_ (1983) covers time keeping from the
invention of the sundial in ancient Greece to the start of the
quartz revolution. Unfortunately a lot of time keeping innovations
from 1970ish onward are not covered, for example all the "gravity
well" corrected versions of time.
you can get lost reading the definitions of all the different "times"
Good list, and your last sentence above is very true. :-)
Another good reference is http://en.wikipedia.org/wiki/Equation_of_time
I use sidereal and UTC time to annotate my calculated star charts; one
example is here from 2003 when Mars had its closest "approach" to Earth
"retro" in the two filenames above means "retrograde" when the planet(s)
appear to move backwards in their orbits around the Sun; details here:
I calculate retrograde charts that agree with NASA's and some examples
are with respect to my back yard's latitude/longitude since my viewing
uses computerized scope mounts (LX200, LX200GPS, LXD750, Atlas, etc. *):
I have current ones for Venus, Mars, Jupiter and Saturn but I don't put
those online unless I'm referencing them in an astronomy forum. I
could do Uranus and Neptune since those are still visible from Silicon
Valley and it's pointless to do Pluto (even though it's one of my
favorite targets and one of my college profs was its discoverer) since
I've to go to Mariposa or Chews Ridge to see it in a scope due to its
faintness plus I'd have to borrow a Hummer or equivalent to both tote
my 14" scope and get to the viewing sites -- my car's ground clearance
* Here's one example setup at approximately 3am mid-March 2003 in prep
for Mars' approach while I was waiting for it to clear some trees so
I took this picture without ruining my night vision:Loading Image...