Hansen’s Ex-Meridian Tables

19 Apr

April 18, 2019/Day 195

Noon Position: 11 33S  24 42W

Course(t)/Speed(kts): WNW 5

Miles since departure: 26,615

Avg. Miles/Day: 137

 

Not sure if wind fell lighter overnight, causing us to slow down, or if we were slowed by taking the wind aft of the beam. In any case, we were slower. Winds are steady but quite light; rarely over 10 knots.

I should be flying the big genoa, but we are back to wind just forward of the beam this afternoon, and that sail is a bear on a reach. At least that’s my excuse. I’ve not changed sail in so long, I’m not sure I recall how.

A bird. A small petrel: larger than a stormy, smaller than a small gadfly; long, delicate wings. No idea what it was. Only bird in days.

I have been dabbling in a new-to-me set of celestial navigation tables. Published originally in 1930, they are, in fact, not new at all, but were a part of the collection of tables my father used when he was an officer in the American Merchant Navy.

The book is titled HANSEN’S EX-MERIDIAN TABLES. According to the inside cover, dad bought the book in 1944 in Liverpool, when he was serving on the S/S F.H. Newell. It’s been tucked away aboard Mo this entire voyage and was only broken out a few days ago.

The tables have been compiled by a Capt. L. F. Hansen, and his instructions in the preface, written for the professional mariner of the 1930s, took some study on my part but have yielded a simple technique for getting one’s latitude.

As you may know, a favorite sight of the sailor is the noon shot for latitude, “the cornerstone of the navigator’s day,” to quote Tom Cunliffe. The sight is easy as it does not require time to the second of GMT; the calculation is also easy and produces a latitude directly; e.g. 47 degrees 27.7 minutes south.

The calculations for forenoon and afternoon sun shots require more look-ups, more steps, and give you intercepts, i.e. lines on a chart, and where they cross, there you are. All fine and good, but it’s reassuring (and did I mention easy?) to know ones latitude.

The problem is that the noon shot has to happen at your local noon. If you miss it, it’s gone until tomorrow.

Enter Capt. Hansen, whose tables calculate where the sun will be for a time AFTER noon.

So, if a cloud was blocking your sun at noon but the cloud is gone by 12:45pm, you can take your “noon” shot at 12:45pm, consult Hansen’s tables, dig out a correction for degrees and minutes to add to your working, and there you are. You’ve got your latitude, cloud be damned.

I’ve used the book these last six days. It took the first three to get the interpolations down, but now my off-noon sights are nearly as close as those of noon.

The sun is said to be “on its meridian” when it is directly overhead; thus, “Ex-Meridian Tables.”

As far as I can tell, the tables aren’t made anymore. Fell out of favor. Don’t know why.

This article was syndicated from The Figure 8 Voyage

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