The Nautical Almanac onlineJun 25, 2014
To the editor: I recall periodically gaining computer access to the Nautical Almanac’s daily pages by simply typing “Nautical Almanac online” into Google’s search engine. I had always thought the almanac’s publisher, the U.S. Naval Observatory, provided the site. I was wrong. Unable to bring up the pages recently on this particular government agency’s astronomical applications department URL — aa.usno.navy.mil — I contacted the observatory directly. A spokesman, Sean Urban, e-mailed this response:
“The U.S. Naval Observatory continues to publish the Nautical Almanac each year along with our British colleagues at Her Majesty’s Nautical Almanac Office… [But] neither office has ever provided the Nautical Almanac online.” The print edition is widely available from Internet booksellers.
What the Naval Observatory does offer online is astronomical data in various formats. Unfortunately, they are difficult or inconvenient for navigators to use. For instance, when I typed “celestial navigation” into the site’s search box, I brought up “Celestial Navigation Data for Assumed Position and Time.” But to access even limited information for the sun, moon, stars and planets, I had to enter latitude and longitude as well as a specific time. For the sun, it was necessary to supply a daylight hour; for the planets, a nighttime hour.
“The page will only bring up data for objects that are near or above the horizon at the location and time specified,” Urban told me.
Surfing elsewhere on the Web, however, I quickly found two sites that gave access to virtually all the celestial information a navigator could ever need. In the first of these, Erik De Man, a Belgian deepwater sailor and electrical engineer, has assembled tabulated material for sun, moon and planets. The URL of the user-friendly site is www.erikdeman.de. Click on “My Nautical Pages” and scroll down to “Celestial Navigation.” Click on that, then scroll down to any of the various “Nautical Almanac Tables.”
By clicking on the sun table, for example, and scrolling down to “Interactive Tables,” you come to a box marked “Table.” Entering the portal brings up the usual hourly listings of sun GHA and declination for every day of the year. The site also provides “Correction Tables for Sextant Altitudes,” though the sun’s semi-diameter is unavailable.
De Man does not offer star tables as part of the package. “I seldom use stars for navigation ‘en route,’” he told me in an e-mail, “and I wanted to have a more compact almanac for daily usage.”
Another excellent online almanac site is www.navsoft.com/downloads.html. But unlike De Man, NavSoft gives a full range of celestial data, including stars, in one set of daily pages. You browse the site by clicking on “Nautical Almanacs 2014.” Listings appear for GHA Aries; GHA/declination Venus, Mars, Jupiter, Saturn; SHA/declination navigational stars; GHA/declination sun and moon.
In addition, there are data for sunrise, sunset, twilight, and the equation of time. NavSoft does not provide corrections for sextant altitudes, although — and this is the flipside of De Man’s corrections listings — it displays the sun’s combined semi-diameter and parallax in a single table.
As to the accuracy of the two sites, De Man told me he generated his information from a variety of U.S. government sources, including a software package programmed by the Naval Observatory.
Mike O’Dougherty, navigation enthusiast and NavSoft’s compiler, said he calculated his data from algorithms developed by the French Bureau of Longitude. “Given the accuracy of the algorithms to a fraction of an arc-second,” O’Dougherty said, “the data should be almost identical to the U.S./U.K. government publications.”
—Alan Littell is a freelance writer and contributor to Ocean Navigator. He lives in Alfred, N.Y., and is the author of the maritime novel Courage, published by St. Martins Press.