This page allows you to obtain all the astronomical information necessary to plot navigational lines of position from observations of the altitudes of celestial bodies. Simply fill in the form below and click on the "Get data" button at the end of the form.
A table of data will be provided giving both almanac data and altitude corrections for each celestial body that is above the horizon at the place and time that you specify. Sea-level observations are assumed. The almanac data consist of Greenwich hour angle (GHA), declination (Dec), computed altitude (Hc), and computed azimuth (Zn). The altitude corrections consist of atmospheric refraction (Refr), semidiameter (SD), parallax in altitude (PA), and the sum Refr + SD + PA. The SD and PA values are, of course, non-zero only for solar system objects.
The assumed position that you enter below can be your best estimate of your actual location (e.g., your DR position); there is no need to round the coordinate values, since all data is computed specifically for the exact position you provide without any table lookup.
Data can be produced for any date and time from year 1700 through year 2035.
Be sure to check Notes on the Data, located after the form.
Notes on the Data:
Data are shown for the navigational stars and planets only if their computed geocentric altitude, Hc, is equal to or greater than +1 degree at the place and time specified. Almanac data for the Sun is shown if its Hc is greater than -12 degrees, the limit for nautical twilight (this is intended as an aid in judging the brightness of the sky). Almanac data for the Moon is shown if its Hc is greater than -3 degrees; when data for the Moon is shown, a note on its phase appears at the end of the table.
Data are shown for objects above the horizon without regard to whether observations of them are practical. For example, data for stars are shown for either day or night, and data for objects that may be too close to the Sun for observation are also shown.
The GHA of Aries is always shown at the end of the list of objects.
The data are color-coded as follows: Data for solar system objects are shown in red and always appear first in the table. Data for the stars that are listed in Sight Reduction Tables for Air Navigation (Selected Stars) (Pub. No. 249, AP3270, Vol. 1) are shown in blue providing that their Hc values are between 15 and 65 degrees; otherwise they are shown in black. Data for the other navigational stars are also shown in black. Data for Polaris and the GHA of Aries are shown in green.
The altitude corrections are intended for use during sight reduction. For a given object, to obtain the observed altitude (Ho), the sum of the altitude corrections (in the rightmost column) is added to the apparent altitude (ha), which is itself obtained from the sextant altitude (hs) by removing instrumental and dip (height of eye) corrections. That is, Ho = ha + Sum. Then Ho can be compared to Hc to obtain the altitude intercept in the usual way. The altitude correction values strictly apply only in the case where the observations were in fact made from the assumed position, and, for solar system objects, the lower limb of the object was observed. Generally, however, these corrections are weak functions of altitude and can therefore be applied, with some small error, to sights made close to the assumed position. The first of the listed corrections, refraction, applies to sea level observations made under standard atmospheric conditions. The SD correction for the Moon includes augmentation.
The data for Venus has not been corrected for phase; under certain conditions this could displace the center of light up to 0.3 arcminutes.
The tabulated data can also be used for observation planning, where a prediction of the the apparent altitude (ha) may be formed by subtracting the sum of the altitude corrections (in the rightmost column) from the computed altitude: ha (predicted) = Hc - Sum. In many cases, the sum of the altitude corrections is negative, so that ha (predicted) will be greater than Hc.
