Lunar distance presetting

Today I conducted a quick exercise presetting a sextant to the Sun-Moon near-limb distance (no refraction corrections).
Sextant: Astra III Professional with the 7×35 Celestaire telescope
Calculations with Navigation Spreadsheets (screenshots attached)

Date: May 26, 2019
Local time: 9:25 am, U.S. Mountain Daylight Time
Universal Time: 15:25
Location: 35° 53’ N, 106° 19’ W

Sun:
GHA: 51° 59.4’
Dec: N 21° 08.8’
SD: 15.8’

Moon:
GHA: 137° 19.1’
Dec: S 13° 17.7’
SD: 14.8’
HP: 54.3’

Topocentric lunar distance: 91° 07.0’
Subtracting the sum of the two semidiameters: 30.6’
Presetting the sextant to: 90° 36.4’

Then, I pointed the sextant at the Moon and soon the Sun appeared right on top of Moon’s limb, as expected. I did not even check the index error beforehand (the sextant has been sitting in its box for months). The whole procedure lasted less than 15 minutes; doing this write-up took somewhat longer than that. 🙂  A good result overall.sunmoonld_prec

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Almanac spreadsheets in 2019

gp_ut

Comparisons with the 2019 Nautical Almanac Commercial Edition show that our spreadsheets remain sufficiently accurate for the year 2019 without the need for any changes or updates.

venus2019

 

22 February 2015 conjunctions

A recent thread on NavList pointed out that a number of celestial objects appear in close proximity during the second half of February 2015. A quick application of the relevant spreadsheets show Venus and Mars separated by 24.7’, which is less than the apparent Moon disk size. The spreadsheet sailings.xls can be used in this case, since the angle of 1 minute of arc corresponds to the distance of 1 nautical mile on the surface of the Earth.

venus_mars

 

With a Moon crescent in the vicinity of the two planets, it has been mentioned that Uranus is also in the area, separated from the Moon limb by about 2 degrees. While in the first example the parallaxes were essentially negligible (and hence the sailings.xls spreadsheet could be used to calculate the Venus-Mars angular separation), this is not the case for the Moon. Therefore, to compute the lunar distance, there is a dedicated spreadsheet (ld_prec.xls) which takes the viewing location on the surface of the Earth (“AP”) as additional input to account for the parallax effect.

uranus_lunar

Uranus appears slightly to the west of the (almost new) Moon crescent, so the interesting quantity is its distance from Moon’s illuminated near limb. Subtracting the Moon semidiameter (16.4′) from the topocentric centered lunar distance of 2° 15.6′ yields a value that is very close to 2 degrees.

 

Ephemerides for the planet Uranus (along with Neptune and Mercury) are not listed in celestial navigation almanacs, as these objects are not suitable for astronavigation purposes. Nevertheless, we provide almanac spreadsheets for those three planets as well, since their data can be computed from the same VSOP87 planetary theory that we use for the other planets.

Venus-Jupiter conjunction of August 2014

As Frank Reed pointed in a recent NavList posting, Venus and Jupiter appeared close to each other in the morning sky of August 18, 2014, separated by about half a degree (30′, or by about the Moon apparent diameter). A quick use of spreadsheets venus.xls, jupiter.xls, and sailings.xls confirms this fact. The first two spreadsheets provide the planets’ ephemerides. The third one calculates the great circle distance of the bodies’ subpoints (geographical positions) in nautical miles, which is numerically very close to their angular separation in the sky in minutes of arc.

venus_jupiter_Aug2014

 

Additional details can be found on Steve Owens’s blog.

Lunar occultation of Aldebaran

The Wikipedia entry for the star Aldebaran contains the following image:

http://en.wikipedia.org/wiki/File:Occultation.jpg

Based on the information on this page (e.g. image was created in July 1997) and after some trial and error with Excel (see screenshots below) I came up with the following plausible coordinates in time and space at which this image may have been created:

New Orleans area:   N 30º W 90º
UT: July 29, 1997,    10h 08m 30s

This really is only one out of many possible solutions, which I did not investigate further.  I neglected refraction which would have a small effect for such a tiny lunar distance (center-to-center topocentric LD = Moon SD = 15.5′) and the overall achievable accuracy in this exercise (no obviously visible refractional flattening of Moon’s disk).  Parallax is important (center-to-center geocentric LD = 34.4′)

Accompanying data look consistent with everything else:
The Moon age (25 days, “waning crescent”) and phase (23% or about 1/4 illuminated)
Local time (UT-6h) => around 4am, about an hour before sunrise (“predawn”)

The two bodies would have appeared due east at an altitude of roughly 34 degrees.


moon.xls:

moonocc


aries_stars.xls:

aldebaranocc


ld_prec.xls:

ldocc


intercept.xls:

interceptocc

 

(first published on May 22, 2011)