There can be little difficulty in remembering that, at the southern winter solstice, on the 21st of June, the right ascension of the sun is six hours; at the northern autumnal equinox, on the 21st of September, twelve hours; at the southern summer solstice, on the 21st of December, eighteen hours; and at the northern vernal equinox, on the 21st of March, twenty-four hours, very nearly: consequently we may say that the daily increase of the right ascension of the sun, the whole year round, is, on an average, almost four minutes.
If, therefore, I wish to know the sun’s right ascension on the 1st of July, I recollect that at the last solstice, on the 21st of June, it was six hours. From this date to the 1st of July, ten days will have elapsed, which, multiplied by the daily increase, four minutes, makes its accumulation forty minutes, which, added to the six hours of right ascension attained by the sun on the 21st of June, gives a right ascension of six hours, forty minutes, on the day proposed.
Having obtained the right ascension of the sun, I have only to subtract that from the mean right ascension of the two antarctic pointers, α and γ Crucis, which being twelve hours, nineteen minutes, may easily be remembered.
“Do I make myself understood, ladies?”
“Oh, yes!”
“On the present occasion we have to subtract six hours, forty minutes, from twelve hours, nineteen minutes: which will leave five hours, thirty-nine minutes.”
“Exactly so!”
“And that five hours, thirty-nine minutes, is P.M. time, when the Southern Cross will be upright on the meridian, on the day proposed, viz., the 1st of July.”
“Well! this is Christmas Eve; what time was the Southern Cross on the meridian?”