wHEN DID I GET HOME?
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John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.org
www.nyskies.org
2011 August 21 initial
2015 September 12 current
It rained and rained during the NYSkies Seminar on August 19th,
2011. The rain let up before we went for supper but clouds still
covered the sky as we parted ways for home.
Recall that the Seminar on August 19th was a discussion of
astrolabes, On the way home I had with me a couple show-&-tell
astrolabes, including my own home-built wood model. That I built, with
my father's assistance, in summer 1959 and saw its first astronomy
service at the october 1959 solar eclipse.
It was still cloudy in Brooklyn when I got out from the subway and
waited for my bus. While waiting, the sky started to clear. The clouds
parted, letting the Moon shine thru. Just below the Moon was a bright
star.
There is no star near the ecliptic bright enough to be so obvious
as this one next to the Moon, It was no star; it was planet Jupiter.
I know, by seeing Jupiter earlier in the year, that the planet is
in Aries about due south of Hamal (alpha Arietis). I did not see Hamal
for the thin clouds over the Moon and the glare from street lights.
My bus arrived. I sat in a front seat to look out the windshield
at the Moon. The bus for part of its route headed east, into the Moon.
The Moon and Jupiter, from familiarity with the local street
layout, were almost at east, azimuth 80 degrees. Hamal, a little to
the celestial north, would have been at azimuth 75 degrees.
Hmmm, what time was it while I was riding the bus to home? I could
have looked at the time on my cell phone. Wait! This is a problem for
the astrolabe!
What did I know?
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Jupiter & Moon were south of Hamal. This star is far north
of the ecliptic. The Moon can not pass north of it.
I allowed that all three were in the same ecliptic longitude.
This wasn't certain because the Moon moves rapidly to change her
longitude substantially within hours.
Because the Moon had to be just south of Hamal, maybe in
conjunction, I can pass up trying to mark Jupiter or Moon on the
rete of my astrolabe. I banked off of Hamal, which is plotted.
Hamal, being a bit north of the Moon, was in about azimuth
75 degrees.
I was in the New York latitude, north 40 degrees, that for
which the astrolabe was built.
The date was August 19th or maybe August 20th if after local
midnight.
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I pulled out my wood astrolabe, it being larger with bolder lines
and markings. It was easier to read on the bumpy bus. I placed the
regula over August 20 on the mean Sun scale and held it there with a
paper-clip. August 20 was a labeled date, with no need of
interpolation. The hour I finally determine would be close to the 0h
of this date because when I was riding the bus it was already very
late in the night of august 19th.
The astrolabe was now set for the date in mean solar time. If I
set the Sun at its date along the ecliptic scale, the hour would be
apparent solar time, which wanders earlier and later than mean solar
time during the year.
I then rotated the rete-&-regula together to put Hamal at azimuth
75 degrees. As a quick check, the ecliptic, near Jupiter did sit at 80
degree azimuth. The astrolabe was now set for both date and a known
aspect of the stars.
The regula pointed to 22:40 on the hour scale of the mater.
Because the regula was now tracking the Sun, this hour is also the
mean solar time of the instant date. The hour of the observation was
22:40 mean solar time, one hour andtwenty minutes before the midnight
between the 19th and 20th.
Not quite.
This hour is in standard time. In August we are in daylight
savings time, one hour ahead of standard time. Astronomers ignore
daylight time but for this exercise I did want to prevailing clock
hour. I was heading home on my bus at 23:40 EDST on August 19th.
You do ask what other riders thought of this gadget, as much an
dart board or game spinner. I explained to the lady next to me that
it's a wireless tablet computer. That seemed to satisfy her curiosity.