The declination of Polaris

Polaris, the Pole Star, or a Ursae Minoris, is not situated exactly at the northern celestial pole. As a result of the precession, the star is now drawing closer to the pole — or, more precisely, the pole is approaching the star.

Table 50. A gives the right ascension and declination of Polaris for some epochs, referred to the mean equator and equinox of that epoch. In the last column the star's north polar distance, 90° minus the declination, is given in arcseconds. In the first column, the decimal zero after the year, for instance 1850.0, means that the epoch is the beginning of that year. More precisely, the unit of time used here is the Julian year of 365.25 days, and the reference epoch is

2000.0 = Julian Day 2451545.0 = 2000 January 1.5.

For instance, 2110.0 means 110 x 365.25 days after 2000.0, that is, Julian Day = 2451545.0 + (110 x 365.25) = 2491722.5, corresponding to 2110 January 2.0. Similarly, the epoch 1800.0 is found to be actually 1799 December 30.5, not the beginning of January 1st of A.D. 1800.

From the table, we note that the right ascension of Polaris is now increasing at an ever increasing rate, and that the declination will reach a maximum around A.D. 2100, when the star will be closest to the pole. An accurate calculation shows that the greatest declination, +89°32'23", will be reached in February 2102. Then the star's north polar distance will be 27'37", or 1657".

In March 1944 the declination crossed the value 5 = +89°00'00". Figure 50.a shows the motion of a UMi with respect to the northern celestial pole.

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