On 1/21/19 5:50 AM, Magnus Danielson wrote:
Hi,
On 2019-01-21 08:07, Mark Sims wrote:
While on the subject of the accuracy/reliability of various algorithms
and web pages showing various astronomical data, we had a full moon /
total lunar eclipse in the northern hemisphere. And not just any full
moon, but a Super Blood Werewolf Zombie Apocalypse full moon (or some
such drivel spouted by all the TV stations).
Anyway, I wanted to know when the eclipse was at it's maximum. Most
web sites gave a time here as 23:12, some differed by several
minutes. None gave the time to the second. That just won't do for a
moon worshiping time-nut, will it? So, I tricked up a version of Lady
Heather to do a screen dump when the difference of the sun and moon
azimuth and elevation were at a (180 degree) minimum. Looks like it
happened at 23:12:04
Well, the best time as we see it is actually about a second after it
actually occurred, due to the time it takes for the light of the moon to
reach us. So the time for the actual event and the time for observation
becomes notisable different.
Cheers,
Magnus
Magnus, this is time-nuts.. "about a second"?
There's a whole bunch of factors that need to be taken into account.
We are close to lunar perigee, where the distance is 357344 km - 1.2
seconds (approx, see below), were we at apogee in a couple weeks 406555
km that would add 150 milliseconds (approx).
Here in Southern California, it was about 9PM local, so we were farther
away from the moon than folks on the US East Coast watching at local
midnight. Why that's almost 2000km - 6 microseconds. (Forgive my
"approximate" here - the intent is to form an 1 sig fig estimate of the
potential error - if needed, we can discuss making this more precise)
Solid Earth tides (assuming the observer is on land)will lead to a few
nanoseconds difference as well you might be closer or farther. For the
lucky folks watching at moon set or moonrise, this effect will be
minimized, since the tidal motion is orthogonal to the line of sight.
For water borne observers, the tidal motion is more complex, and you'll
have swells to worry about as well.
One should also take into account the density and properties of the
atmosphere, which changes the propagation speed of light. One should, of
course, also account for the dispersion - red light travels at a
different speed than blue - but perhaps the red moon was sufficiently
monochromatic that this isn't an issue?
As it happens, it was overcast here, so there was no direct viewing
possible, so my viewing, such as it was, had to be mediated through the
internet, which has its own set of time lags, some of which we have
discussed on the list.
In all seriousness, computations of the observing times of astronomical
events does need to take a variety of factors into account, and light
speed is but one of them.
In my own professional field of spacecraft telecom, as beamwidths get
narrower (i.e. going to optical wavelengths instead of RF) knowing the
precise relative position at a specified time is important. This is
biting lots of "cubesat" folks as they move from UHF with 30 degree
beamwidths to ground station antennas with 1 degree beamwidth.
And, when looking at links to outer planets, with transit time in hours,
do you need to "point ahead" (earth moves about 30km/sec in its orbit) -
from Mars at a sort of average 1AU, that's about 0.007 degrees.
Probably not an issue with a 1 degree beamwidth antenna.
A typical 20cm optical aperture will produce a beamwidth of 1500 km at 1
AU, a bit less than 1 minute's motion of Earth.
One thing I'll note is that for the most part, folks realize the
impracticality of computing all this stuff open loop - you do a
calculation to get you close enough, then use closed loop tracking to do
the final pointing. But think about this - if I'm pointing my telescope
back at Earth, finding Earth is easy. Pointing to the centroid of Earth
is easy. But my "spot" is only 1500km wide, on a 14,000km wide earth, so
I need to know "where on the Earth disk" to point.
http://kiss.caltech.edu/workshops/optcomm/presentations/Sburlan.pdf is a
nice high level presentation on Optical Comm from a few years ago.
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