Astron user notes ( 14)

1. INTRODUCTION

• 
  As an almanac, Astron will, for an entered date and time, calculate the GHA, Dec, HP & SD of a body selected from a list of the 57 navigational stars, Polaris, Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. In context, it also calculates Sun and star rise and set times, Moon phase, body magnitudes and twilight times. Although intended for practical use in the present era, dates between 1905AD and 2999AD may be entered.
  
• 
  If an assumed position is also entered, Astron will also calculate the Azimuth and Altitude of the selected body. (If the body is unidentified, a sidebar utility can identify it given its approximate azimuth and altitude.)
  
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  If a sextant altitude is also entered, together with instrument correction, height of eye, temperature, pressure and observed limb, Astron will also calculate the Azimuth and Intercept from the assumed position.
  
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  Several supplementary features, such a sight planner, are also included.
  

Settings: Ship’s Time Mode, Time Zones, Metres, Celsius and hPa.

Date/Time: 2017 June 05th. 20:05:46 ship’s time. Watch Correction 0, Time Zone -5, Daylight Saving 0.

Body: Spica. Assumed Position: 40N 75W.

Unidentified body: Approximate bearing 320ºT and altitude 10º.

Sextant: Index Correction +0.6. (0.6 off arc.)

1. 
   Spica sight. For this observation time, Astron calculates the magnitude, GHA and Declination of Spica, followed by the azimuth and altitude of Spica as would be observed from the assumed position at that time. Given the subsequent sextant reading, index correction, eye height, temperature and pressure, Astron calculates that the azimuth and intercept were 173.2º and 10.3nm Away. Thus, the observer was somewhere on a line through a point 353º / 10.3nm from the assumed position drawn in direction 083º/263º.
   

2. 
   Advance line of position. (First supplementary block below main display.) If you took this sight as one of several for a fix at 21:00:00 and your vessel was tracking 060° at 6 knots, you would need to advance the position line in direction 060º for 5.4 nm. (Alternatively, plot the azimuth and intercept from N40° 02.7' W074° 53.9').
   
3. 
   Twilight. The observation window that evening was between 19:40 and 20:19 ship’s time. (If the Sun had been the selected body, this area would have displayed Sun rise and set times.)
   
4. 
   Unidentified body. That body about 15° altitude in the North West is Capella.
   
5. 
   Arc distance. The calculated observed arc distance between Capella and Spica, allowing for refraction, on this date was 120° 07.16’.
   
6. 
   Observed Lunar Distance. Astron calculates the observed lunar distance between Spica and the near limb of the Moon at the entered time and location as 14º 38.4’.
   
7. 
   Lunars utility. (Third block below main display). If you had been at the Assumed Latitude of exactly 40N and had measured the angle between Spica and the near limb of the Moon with your sextant (IC=+0.6) as 14º 37.8’, your longitude would have been W074º 59.0’ and the ship’s time would have been 20:05:42. (Not really a valid lunar example as the iterative process and longitude synchronisation with a second body have both been omitted. Also, for lunars, you should really work in GMT mode. See Section 18 for the full method.)
   
8. 
   Upper Meridian Passage. (The pop-up refers you to the second block below main display – Upper Meridian Passage.) If you had been stationary taking sights of Spica before and after it reached maximum altitude and had entered Hs as 38º 23.4’ as the maximum observed altitude and 20:05:46 as the (deduced) time of maximum altitude, then your position at that time would have been N40º 25.2’ W069º 33.2’. However, if you had been moving at 060º at 6 knots, you would have been at N40º 25.2’ W069º 45.0’ at 20:06:33.
   
9. 
   Sight Planner. (Separate sheet accessed using tabs at bottom of screen.) Should you wish to use this Spica sight with other bodies for a fix, the best cut for a two-body fix would be from Spica and Rasalhague, though you would probably choose Regulus as it is brighter. For a three-body fix, you would use Spica, Vega and Pollux. For a four-body fix, the lack of bodies suggests that you should try a different ‘reference body’ to Spica. (The obvious ones are the Moon or Jupiter, but Spica was necessary to trigger all the various pop-ups!)
   
10. 
    More on lunars. (Still on sight planner sheet.) If you are a lunar freak, you would further observe that Jupiter, Denebola, Regulus and Pollux would also give lunar opportunities. (Zuben’ubi is too close: zodiacal bodies are indicated by a period (.) after their name.)
    
11. 
    Star rise, upper meridian passage and set times. (Now on computer almanac sheet again, lower right.) Spica rose at 15:03:23, was at maximum altitude at 20:27:30 and set (earlier that morning) at 01:55:32. These are theoretical times. Section 17.1A of these notes points out that, because of atmospheric extinction, Spica would not be visible to the naked eye below altitudes of about 1° 48’.