Friday, October 25, 2019

Air Almanac Compared to Nautical Almanac

We have recently expanded this article with a short booklet covering all the details. In the meantime,  several major points of comparison are outlined below.

_________________________

For routine cel nav we recommend using the Nautical Almanac (NA) for the needed cel nav data and corrections. Government issues of this annual publication are expensive ($52), but one company is licensed to print and sell a copy for less, called the "Commercial Edition" ($30). There are less expensive (even free) alternatives to the Nautical Almanac, but we still recommend the official NA over various homemade versions, primarily to standardize the teaching of the subject, which is the same reason we do not make up logical names or abbreviations for various parameters that have traditionally unclear or even misleading names—i.e., zone description is called ZD, zenith distance is called z, azimuth angle is called Z, and azimuth is called Zn, to give one example.


One such official option (not of the homemade variety) is the Air Almanac (AA). This book is produced by the same agency as the Nautical Almanac (Nautical Almanac Office of the United States Naval Observatory, USNO), and includes pretty much the same data, but this one is a free PDF download.


An important side note here is the USNO has gone offline for 6 months starting Oct 24, 2019, so the fate of the 2020 Air Almanac is still up in the air! The reason given is to redo their website, which must imply a serious issue, since this is not the standard way to do such things.

You can get the 2019 edition from this link


You have to set up an account, then buy it at no charge, check out, then get an email with a link to the download. Perhaps we can change the date to 2020 and later get that issue; or do a search on "Air Almanac" and see if it is listed. Normally the following-year editions of both NA and AA are out by Sept or Oct. The NA is out now; the AA is not.

[Note on 2020 edition added on Dec 15, 2019 is at the end of this post.]

With that long introduction, we can proceed with some features of the AA. First, the file you download will have sections out of order.  It starts out with the "daily page" data; the cover page and introduction follow that—seems the subsections sorted in a way not intended. There are bookmarks in the file, however, so open the bookmarks panel on the left in Adobe Reader or Acrobat to see what is where.

The basic GHA and declination data are essentially the same in both publications, although the layout is different.  Below are sample daily pages from the Nautical Almanac (NA).


In the NA, we see sun and moon on one page, along with sunrise/set and moonrise/set.



In the NA, planets and stars are on the facing daily pages.


In contrast, the AA has only one daily page, with a different layout, as shown below.

A sample daily page from the Air Almanac

In the NA, we get GHA and declination every hour, and then we correct for the minutes and seconds using the NA's Increments and Corrections Table.  In the AA, we get this data every 10 minutes, and make the minutes and seconds corrections using the AA's Interpolation Table. Both make the same level of corrections; values for the sun in each book will be the same ± 0.1'. Notice too that the day of the year (DOY) is listed on each daily page of the AA, whereas in the NA, DOY values are in a separate table. DOY is convenient for figuring watch errors and for ETA computations over a long ocean crossing.

There is some subtlety here, however, in the presentation. Notice that Venus is listed in the NA but not the AA. Normally whenever Venus is available it is a good object to shoot, because it is so bright (planet magnitudes are listed next to their names) we see it early evening twilight before the stars are visible, or later in morning twilight after the stars have faded. In short, it extends the sight taking time period. The reason it is not shown in the AA is it is not a practical target at this time, because it is too close to the sun—compare sun and Venus GHA and dec in the NA data above. To be a useful evening or morning star, it has to be far enough from the sun that is is not buried in bright twilight; a point we look into below. So the AA just does not list it.

For the other planets, the AA will likewise not present data when they are not useable for cel nav, as shown below.


The same thing can happen with the moon, which is presented as follows:


In a sense, then, the AA is a bit more user friendly to navigators as it includes this extra bit of information.

A big difference in format is the AA only gives moon and planet data to the nearest whole minute, which effectively is rounding from 0.5'.  The overall guaranteed uncertainty of the values are slightly better in the NA than in the AA, both of which have sections discussing Accuracy. This discussion in the AA, however, is more pessimistic regarding final fix accuracy because they are assuming sights from an aircraft. This could be a handicap for those with good sextant skills, taking the right sights in good conditions, in which case they could lose a few tenths of a mile accuracy in some sights. It could also affect gyro bearing calibrations where we do indeed want azimuths accurate to the tenth. But for all practical purposes, the precision of the AA is plenty adequate for routine cel nav on any vessel, and indeed the AA does include a Polaris azimuth table accurate to the tenth of a minute, just as the NA does.

The latitude by Polaris correction in the AA is presented as a single Q correction, which combines the a0, a1, and a2 corrections used in the NA. This Q-method is a bit quicker to implement. (We use this method in our book GPS Backup with a Mark 3 Sextant.)

The AA does not include the polar view and tropical Mercator star charts included in the NA, but it does have a star chart of its own that all navigators can benefit from. It is often presented on its own, because it uses navigational star id numbers, so we just note that the source of this well known star chart is indeed the AA.

Sun and moon rising and setting data as well as twilight and LAN times are about the same in both books, although they are laid out differently.

The AA has a better list of symbols and abbreviations, or at least it can be considered a nice supplement to the NA data. Both books include a list of places on various time zones and who uses daylight savings, and both include similar Planet Location diagrams, with the AA versions, if anything, easier to use.  Both have an Arc to Time Table. The AA includes a few tables unique to aircraft sextants and some unique to high latitude flying, neither of which are relevant to marine navigation.

Sky Diagrams

The AA does include one unique set of data they call the Sky Diagrams. These can be used to choose the best sights, combining both stars and planets during twilight, as well as best times for sun-moon fixes during the day. They are meant to be used by inspection alone, without special computations.

This method is in principle better than using Pub 249, Vol.1, which tells the best 3 stars to use in any situation, but does not include planets and does require use of the NA. Also in principle it could be better than using the 2102-D star finder, which can be set up to include both stars and planets, but it takes some prep work to do so, and it too requires a NA. A sample of the Sky Diagrams is shown below.

Sky Diagrams from the AA: Evening sky at Lat 25N, July 15, 2019, 17h left and 19h right, LMT.

These diagrams take some explanation, which is presented in-depth in a separate article, comparing this method of sight planning to the several options available. It is an interesting exercise. North is at the top of the diagram; the circumference is the horizon; the center is overhead; each ring is 30º of altitude.  I sketched in a course line of 225T.

In these pictures, the numbers represent the navigational stars, each of which has a unique number; the letters are the planets; circle with center dot is the sun; circles with a date inside is the moon on the dates shown; NP is the north pole of the sky, which will always be at a height equal to our Lat; in this case 25º.

We see that at 25N on July 15 at 17h LMT, the sun is just about to set to the north of west, with Venus (V) preceding it—thus it will be a morning star the next day— and Mars (letter M) following it over the horizon as an evening star. The moon is not visible (or maybe right on the SE horizon, we can't quite tell from this). With the sun up the stars and planets showing would also not be visible.

Two hours later, the moon is likely on the beam, low on the horizon, with Mars being an evening star about 25º high, located 20º north of west. The star Regulus (#26) is due west, about 30º high, broad on our starboard bow.  And so on...

With this type of presentation, we can apply the basic principles (location, height, and brightness) to choose the best triad of bodies for the evening sights.  It will not matter that we are not there at precisely 17h or 19h LMT, nor that we are not there on precisely July 15, because the bodies will not have moved much, relative to our criteria for choosing the optimum triad. The arrows in the 17h plot show how much the stars move during 15 days, before or after July 15.  I will illustrate these issues in the next article on actually choosing sights this way and comparing pros and cons of the several ways we have to do this.

The Sky Diagrams take up 71 pages of the AA. There are another 23 pages of "Polar Sky Diagrams" that are intended for navigation between 75N and 90N. This is common ground for aircraft, but not marine craft. These do not have application to typical ocean navigation, but these views of the sky when standing at the North Pole are interesting on their own—our planetarium talks on star ID, for example, start at the North Pole.

Many small craft mariners are pleased that this Air Almanac, as well as the Pub 249, Sight Reduction Tables for Air Navigation, are still in print today. Some mariners find these preferable to the corresponding marine texts, Pub 229 and the Nautical Almanac. Those using the AA for marine navigation will want to get a copy of the altitude corrections for sun and moon from the NA. All else is about the same, or maybe easier with the AA, but those correction tables are needed.

With that brief comparison of the two almanacs, I want to stress that we still strongly recommend the Nautical Almanac for routine cel nav. Besides the standardization point mentioned at the start, the NA also includes a complete set of sight reduction tables (not in the AA), which makes it a one book solution to celestial position fixing. It also has much more discussion of the tables themselves, including, for those who want it, analytical solutions that can be used in personal calculators or computers.

Even more to the point, once one has learned traditional cel nav using the NA and sight reduction tables, including the NAO Tables from the NA, we recommend buying the NA, double zip-lock bag it, and store it in a safe place on the boat. Then do your cel nav planning and analysis with some version of the StarPilot, which will yield much faster and more accurate results. You will not need any other tools or books.

Note on 2020 edition added on Dec 15, 2019

The 2020 edition has been produced but is not yet on the Gov Bookstore site. We expect it to be there shortly and it will sell as a CD for $28. It is not clear yet, if the free PDF download will be there. We will update this note when we know.  In passing, the CD version is not just a copy of the 48-MB single PDF we could download for earlier years, but rather it is a combination of 53 separate PDF files along with 7 HTML web pages that serve as an index to the parts.

_______________
Dec. 15 update: 2020 edition is now available at the NavList