Almanac spreadsheets in 2021


Comparisons with the 2021 Nautical Almanac Commercial Edition show that our spreadsheets remain sufficiently accurate for the year 2021 without the need for any changes or updates in the calculations.

Spreadsheets aries_stars.xls and what_star.xls were modified with the changed spelling of one star’s name (Alnair). Archive TAR and ZIP files were also updated accordingly.

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

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

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

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.



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 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.



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:

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.










(first published on May 22, 2011)

Lunar eclipse, December 2010

Here is some info pertaining to today’s lunar eclipse.  You can see that the Earth is indeed right between the Sun and the Moon, and that the eclipse will be nicely visible (weather permitting!) from western United States.

1) On the Sun-Earth-Moon positional arrangement:
a) the Sun and Moon declinations are very close to being equal in value and opposite in sign/hemisphere, and,
b) their Greenwich Hour Angles (GHA) differ by 180º, which places the two bodies on opposing meridians.

The Moon is of course 15 days “old” and its phase is “full” (100% of the disc illuminated).

2) On the visibility of the eclipse:
The Moon subpoint (a.k.a. GP = geographical position) is:
Latitude (from declination): N 23º 44.9′ (very close to Tropic of Cancer)
Longitude (from GHA): 124º west of Greenwich
This location in the Pacific Ocean rather close to Baja California makes this eclipse visible from our area.

The Sun is essentially on the Tropic of Capricorn (declination S 23º 26.2′) making this a rare event when an eclipse coincides with a solstice.

The time given is Greenwich time (Universal Time, UT) which is 8 hours ahead of our own Pacific Standard Time.



A picture taken by a friend of this blog:



(first published on December 20, 2010)