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What Day Is It In Greenwich - PeterB - 04-08-2022
In a much earlier post on this forum titled “A Celestial Navigation Problem I Can't Solve”
There was discussion regarding the determination of the correct Greenwich day and date. The confusion arises because Chronometer Time (CT) is given as 9:20:05 but this doesn't indicate if this is AM or PM in Greenwich. Apparently this is a common way to state problems on rating exams and it is up to the individual to figure out if it is AM or PM by other information included in the problem.
In this case it could be 9:20 AM in which case it is 09h 20m 05s ( 2 2/3 hours before noon in Greenwich,) or it could be 9:20 PM which would be 21h 20m 05s ( 9 1/3 hours after noon in Greenwich.) The DR longitude is given as 163° 51.0' E
The solution to the puzzle is to draw a time diagram.
Time diagrams are a schematic representation of the Earth with the locations of pertinent information sketched in. We think of them these days in terms of angles of longitude, but before about 1940 these were all done in units of hours and only converted to angles at the last.
I like to draw my time diagrams as if you are looking down upon the Earth from above the north pole. This way the motion of the celestial objects progresses clockwise with the passage of time. A lot of people prefer to draw them as if you are looking up from beneath the south pole in which case the motion of celestial objects is counter-clockwise as time advances. It is just a matter of personal preference but be sure you pay attention to which convention is in use and to mark your own diagrams with which way is westward.
Fundamental to understanding how to determine the day and date in Greenwich is to realize that the “new” day extends from the International Date Line (IDL) all the way around the world to the midnight demarcation opposite the sun. As midnight moves westward the new day expands around the world with it wiping the old day away ahead of its advance. When midnight gets back to the IDL the next new day materializes, starting out as a mere sliver, but expanding with the motion of the midnight demarcation.
I simplify my diagrams just to get “the big picture” and fill in the details afterward. If you are near to the real date line it zigs and zags around a great deal, but this method will still get you in the right frame of mind and you can then make adjustments accordingly.
In the same video referenced in the post “Celestial Problem I can't Solve” there is another example of having to determine the day and date in Greenwich from ancillary information. This time a time diagram is not as much help.
The date is March 11, 1981
The DR position as 23° 31.0' N 62° 25.0' W
The sight is of Kochab in Ursa Minor (The Little Dipper) at Hs of 10° 41.1'
Chronometer Time is 9:48:43 with no AM or PM stated
Since Kochab is only about 16° from Polaris it is circumpolar for most observers in the northern hemisphere including at this DR latitude.
In this instance the zone descriptor is +4. That makes it pretty clear that Greenwich is in the same day as the local day, but is it AM or PM? The local zone time could be either close to 6:00 AM or 6:00 PM and either one could be believable as twilight – so that alone is not much help. Instead here is what I did:
The altitude of Polaris should be very close to your latitude, so from this DR Polaris would have an Hs close to 23° Since Hs Kochab is much lower than Polaris [being close to 11° ] this tells me that its geographic position (GP) must be over the other side of the world from me.
The SHA of Kochab (from the 1981 almanac for November 11) is: SHA 137° 18.3'
The two nearby possible listings for the GHA Aries are:
at 10h 00m 00s 319° which gives me a GHA* of 95° and LHA* of 33°
and
at 22h 00m 00s 139° which gives me a GHA* of 276° and a LHA* of 214°
If the LHA were 33° I would have observed Kochab above Polaris, so the correct time to use must be closer to 22h 00m 00s which in this case is CT 21h 48m 43s
Once again it is helpful to round to whole hours and degrees to simplify the problem. Usually that is close enough to work