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.

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Mercury in January 2015

In January 2015 Mercury is visible just northwest of Venus in the evening sky. Its horizontal parallax (HP) is twice that of Venus, so current Earth-Venus distance is about twice the current Earth-Mercury distance. 2015 Nautical Almanac Commercial Edition mentions the two planets in its “Do Not Confuse” paragraph on page 8. Spreadsheets mercury.xls and venus.xls show that the geographic positions (GP) of the two planets are very close to each other at this time.

mercury_jan2015

venus_jan2015

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.

Five years of Navigation Spreadsheets

Navigation Spreadsheets logo
Navigation Spreadsheets

At the fifth anniversary of our website’s launch we review some of Navigation Spreadsheets functions.  All three examples are taken from the 2014 Nautical Almanac Commercial Edition.

1) Ephemeris (almanac data), Venus GP on 2014 May 5 at 13h 15m 18s (p. 256):

venus.xls

Input:
UT: 2014 May 5, 13:15:18

Output:
GHA = 58º 58.0’
Dec = S 0º 14.1’

venus2014

 

2) Sextant altitude corrections (Venus, p. 259)

alt_corr_xls
Input: Hs = 4º 32.6’
Output: Ho = 4º 17.6’

altcorr2014

 

3) The calculated altitude and azimuth (pp. 279-280)

intercept.xls
Input:
GP: GHA = 53º    Dec = S 15º
AP: Lat = N 32º    Long = W 16º
to which we add Ho = 30º 30.0’ in order to allow the calculation of the intercept and the plotting of the LOP.

intercept2014

 

Output:
The resulting LOP (intercept 38 NM away, azimuth 223) is plotted with the T-Plotter.

tplotter1

tplotter2

 

As it was also calculated by intercept.xls this LOP crosses:
the AP’s meridian at 52 NM north of the AP
the AP’s parallel at 56 NM east of the AP

tplotter3

 

(first published on February 15, 2014)

Example of a great-circle route

In a recent NavList thread titled “Great Circle Puzzle” it is revealed that a great-circle (i.e., straight, or, direct) sailing path exists between Pakistan and Russia.  This may seem impossible based on a quick look at the world map.  However, several NavList contributors established the end points and provided a general description of such a path.  Using these results it is possible to calculate this path in detail with sailings.xls and waypoints.xls spreadsheets:

waypoints.xlswaypoints

 

If you have a globe handy, check it out!

gc12

gc34

For additional details see this post and the entire thread.

World map credits:
http://www.mapsfordesign.com
http://www.bjdesign.com

(first published November 3, 2013)

Almanac spreadsheets in 2014

Selected comparisons with Nautical Almanac 2014 Commercial Edition indicate that almanac spreadsheets are good for the year 2014 as they are – with one small update described below.

The newest versions of aries_stars.xls and what_star.xls have the computed SHA of Rigil Kent. increased by additional 0.3′ before display (the original adjustment had been +0.6′, so now it is +0.9′, see cells W305 and W306) in order to better match with published almanac values.

A possible explanation for this oddity has been suggested by Antoine M. “Kermit” Couëtte in his recent NavList post.

Downloads of all (including updated) spreadsheets are available on our main page.

(August 31, 2013: Additional details were provided on the same NavList thread by Paul Hirose and Dave Walden.)

 

(first published on August 25, 2014)