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January 13, 2011, 1200 GMT, sun's declination is S 21� 29.2'. At 1300 GMT it is S 21� 28.8'. The change in declination for that one hour is 0.4'. If you read down the column of sun's declination you will see that it changes by 0.4' every hour so that is the value written for the "d" correction factor at the bottom of the column. Note that there is no sign attached to the "d" value so you must look at consecutive hours to ascertain if the numbers are getting bigger or smaller. There are "d" values given for the planets and the moon also and the correction is made in the same way for them except the "d" for the moon is tabulated for each hour as it changes rapidly. Used in the same way is the "v" corrections for the planets and moon to account for the changes in GHA that differ from the standard used in making the tabulated GHA values in the increments table 15� 00.0' per hour for the sun and planets and 14� 19.0' for the moon. Also note that the "v" correction may be positive or negative and the sign is included in its tabulation.

When you go to the "increments and corrections table" recognize that the three left columns are the "increments" table and the three right columns are the "corrections" table. The increments are the tabulated changes in GHA for every second of time between the tabulated hourly values and march along like clockwork. The "corrections" part of the table is simply a multiplication table that multiplies the "d" or "v" value by the time in minutes only for the time that has elapsed since the tabulated hourly values of declination and GHA.

So to find the coordinates of the sun for 12:30:30 Z on January 13th you look first at the first first column for 30 minutes and 30 seconds and take out the increment of 7� 37.5' which you add to the 1200 Z tabulated GHA of the sun. To find the declination you look in the right most columns for the "d" value, .4', and then take out the correction adjacent to it which is, not surprisingly, .2'. If you compare other time intervals you will find that a "d" value of .4 will result in a correction of .2' for any time between 22 and 36 minutes.

One more thing, don't confuse this "d" correction for the "d" correction used in sight reduction tables such as H.O 214, H.O. 229 and H.O. 249.

gl

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