[AZ-Observing] Re: Space Station Lunar Transit Picture
- From: gene lucas <geneluca@xxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Tue, 15 Apr 2008 16:50:33 -0700
Don't forget to include a correction for the observer's elevation....
For example... Most occultation predictions are calculated for an
observer position at the center of the Earth... The allowance for
elevation is termed the "Tan Z" correction. It produces a shift
proportional to the observer's elevation and >perpendicular to the
track< (in the direction of the Moon, for instance), or in other words,
a >South< shift for observers in the Northern hemisphere, and vice-versa
for the Southern hemisphere.
For example, the Tan Z correction amounted to over a quarter mile south
shift for a lunar graze we observed south of Denver a few years ago
(from ca. 6,000 ft elevation near Castle Rock).
While I am most familar with the effect on Lunar graze track
predictions, this might also affect the satellite track predictions from
CalSky, and will be proportional (according to the tangent) for
increasing observer's elevation... It would work the same way as for
the Moon (which is also in Earth orbit....)
We are already at ca.1,100 ft elevation (MSL) in Phoenix, and the shift
is not insignificant. It would be very much stronger if you went to a
position, say, at the top of South Mountain (another 2,000 ft above the
valley) or even in Tucson... which is at an elevation similar in height
to the top of South Mountain, right in downtown Tucson. Higher yet on
top of, say, Mt. Lemmon...
Tom Polakis noted a small shift from the CalSky track at his position
(near the foot of South Mountain) for the latest ISS pass...
Would be interesting to collect video from two separated positions, and
produce a combined 3-D (Red-Green analglyph) image of a satellite pass.
Gene :Lucas
(17250)
Dan Heim wrote:
>FYI, the parallax can be calculated from a long skinny right triangle with
>its vertex at the ISS (assuming the Moon at infinity and ISS at 350 km). It
>goes like:
>
>tan(parallax)=(horizontal displacement)/(350 km)
>
>So, for example, a half-degree parallax (the width of the full moon),
>translates into:
>
>horizontal displacement = tan(0.5) x (350 km) = (0.0087) x (350 km) = 3 km.
>
>Earth curvature introduces a slight error into this calculation, but it's
>not significant for such small angles. I watched the event from New River,
>and the ISS appeared to pass about 5 degrees south of the Moon. Excellent
>image Tom! Wish I coulda' been there.
>
>Dan Heim
>President
>Desert Foothills Astronomy Club
>http://www.dfacaz,org
>
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