Sunday, June 22, 2008

Corrections To Apparent Altitude

Index correction and dip correction depend on the instrument and the height of the observer. The remaining corrections depend on which body was observed and the altitude obtained. Listed below are the various corrections.

1. Refraction:
A correction made to the altitudes of all bodies is the correction for refraction. Refraction occurs because the light rays from the body encounter a denser medium, the earths atmosphere, and are bent downward.

Refraction causes bodies to appear to be at greater altitudes than they are actually at. For this reason the correction for refraction is always negative.

Refraction correction is maximum for bodies at altitudes near the horizon. (zero) and decreases as altitude increases. Bodies at the zenith (90°) do not require a refraction correction. The main part (left half) of the "STARS AND PLANETS" correction table inside the front cover of the Almanac is a table of refraction corrections for various apparent altitudes from 9° 56.0 and upwards. Like the dip correction table, the main refraction table is a "critical table" and no interpolation is required.

Refraction values for altitudes less than 10 degrees for the sun, stars, and planets are located on the next page (A3) of the Almanac. This is not a crit­ical table, meaning that interpolation is required as necessary.

Due to their great distance from earth, no corrections other than IC, dip. and refraction are required for correcting sextant altitudes (Hs) of stars, Refraction is "built in" to the altitude correction tables for the sun and moon, in that it is mathematically combined with other factors in deriving the values which appear in the tables. This is the case with the sun's val­ues in the low altitude table.

Refraction values in the main tables are for a temperature of 50 degrees Fahrenheit (10°C) and atmospheric pressure of 29.83 inches (1010 mil­libars). If temperature and/or pressure at the time of a sight vary a great deal from these fairly standard values, especially when temperature is below 10°F., an additional correction for non standard conditions should be applied. These corrections are derived from the table on page A4 of the Almanac.

2. Semidiameter:
When using a sextant to measure the altitude of a body, the center of the body is used for measurement. The semidiameter correction is incorpo­rated into the altitude correction tables for the sun and moon because it is impossible to determine the altitude of the center of these bodies with a conventional sextant. For this reason you measure the altitude to the lower limb or upper limb of the body. The choice of which limb used depends on which is visible.

The semidiameter correction is added to lower limb altitudes and subtracted from upper limb observations. In practice, the semidiame­ter correction is included as a component in the correction found in the tables for the respective bodies.

3. Augmentation:
The augmentation effect is the increase in apparent size of the body (which increases semidiameter) as a result of an increase in altitude. This occurs because of the nearness of the sun and moon to the earth, and because the observer is on the surface rather than the center of the earth. Augmentation effect is zero for a body on the horizon, and maximum at the zenith.

The augmentation effect is a component of the main correction in the moon's altitude correction tables. It is not included in the tables for the sun because the effect, though measurable by astronomers, is so small as to be considered negligible in navigation.

4. Phase:
Phase is another small correction which must be applied to altitudes of Venus and Mars. Because the light we see from these planets is reflected from the sun, the center of the planet which we see in the sextant optics may not be the true center. Like phases of the moon, only part of the body may be illuminated depending on where the body is in it's orbit. The phase correction corrects the sextant to an equivalent altitude of the center of the body. This correction is a component of the additional corrections for Venus and Mars found inside the front cover (page A2) of the Nautical Almanac.

5. Parallax:
Last, you must consider the correction that must be applied to observations of bodies within the solar system due to the observer being on earth's sur­face rather than at the center. The difference between the altitudes as mea­sured from the surface and from the center is called parallax. The value of the correction is greatest when the body is on the horizon, called horizon­tal parallax, and zero when the body is at the zenith.

The parallax correction is greatest for observations of the moon, and varies from hour to hour. Parallax corrections for the sun are next in magnitude and are a component of the sun's altitude correction tables in the Almanac. Parallax correction for Venus and Mars is included in the addi­tional correction tables for Venus and Mars.
Note: These additional corrections for Venus and Mars are only applied to altitudes from night-time observations.