Codes

Sample calculation

If you live in New York, NY (USA) your latitude is 74w0 (that is, 74 degrees, 0 arcminutes west of Greenwich). You are in the Eastern Standard Time zone. In winter, your official clock time is GMT (or UT) minus 5 hours. (This would be plus a correction in case of east rather than west latitude.) In summer (remember fall back, spring forward) the correction would minus 4 hours.

But the EST zone spans the band of latitudes from 60w0 (eg, Eastern Maine) to 75w0 (eg, Central New York State) and beyond, to ca 80w0 (ie, almost to Chicago, IL). The TLT depends on latitude, and changes gradually from -4 hours at latitude 60w0 to -5 hours at latitude 75w0, and -6 hours at 90w0. Thus the TLT at New York City is 4 hours and 56 minutes ahead of GMT, which translates to LT plus 4 minutes in summer.

Suppose we need to know then the sun is directly overhead in New York City. At noon clock time (noon EDT = 1700 UT) the sun is directly overhead on the meridian of latitude 75w0, so it passed over New York City (74w0) four minutes earlier. The correction (4 minutes) is added to LT (1156 hours EST) for true local noon (TLT = 1200). Remember: TLT = LT + TLTC (in this case, 1200 = 1156 + 0004).

Our TLT calculator simply translates LATITUDE into TLT. In this case, entering 74w0 into the form returns -4h56m from the calculator. Knowing the LT is UT-5:00 in the winter (EST), you then conclude that the TLTC is +0:04, as TLT = LT + TLTC.

Rev 11 Sep 2003 by Ralph Abraham