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The Challenge: Measure Latitude and Longitude

Compensating for the Earth's tilt

Unfortunately, this reading is only correct on 21 March and 21 September (the spring and autumn equinoxes). At midwinter (21 December) you should deduct 23.45° from your reading, and at midsummer (21 June) add 23.45°. This is because of the way in which the 'tilted' Earth orbits the sun.

solar declination
Solar declination - the seasonal consequence of the 'tilted' Earth's annual movement around the sun.

Although complex tables (almanacs) are used to obtain correction factors for any day of the year, you can accurately estimate the appropriate numbers for yourself. For example, if calculating the value at the beginning of May, you would be half-way between the spring equinox (when the sun is directly over the Equator) and the summer equinox (when the sun is directly above a point 23.45° north of the Equator). You would therefore need to add half of 23.45°, or 11.73°, to your reading.