Astronomy Principles and Practice Fourth Edition.pdf

Hence, the period of the year during which the Sun’s declination δ has values
δ c N ≤ δ ≤ 23 ◦ 26 ′ N
The seasons 103
is the period during which the Sun never sets even at midnight. This is the phenomenon known as the
midnight Sun. It is obvious that June 21st will lie at the middle of this period.
It is easily seen from figure 9.9 that while the Sun’s declination has values between δ c Sand
23 ◦ 26 ′ S, the Sun will remain below the horizon. Again, the numerical value of δ c Sisgivenby
δ c = 90 − φ.
This period of time when the Sun does not rise, known as the polar night, is around December
21st, the winter solstice in the northern hemisphere.
Obviously for a person in latitudes further south than the Antarctic Circle (66 ◦ 34 ′ S),
corresponding phenomena are observed, though the date halfway through the period of the midnight
Sun will be December 21st; the date June 21st will lie at the middle of the time interval during which
the Sun is never seen.
At the North and South Poles, the observer experiences six-month periods with the Sun
continuously above the horizon alternating with six-month periods when the Sun is below, neither
rising nor setting.
9.11 The seasons
Time is also measured according to the passage of the seasons. Spring, summer, autumn and winter
have always loomed large in mankind’s life and philosophy, linked as they are to seed time and harvest,
good weather and bad and the length of the day. They are, of course, also directly connected with the
Sun’s passage round the ecliptic.
In the northern hemisphere spring, summer, autumn and winter are defined to begin when the Sun
respectively reaches the vernal equinox,thesummer solstice,theautumnal equinox and the winter
solstice.
Thus, in spring, the Sun’s right ascension increases from 0 h to 6 h while its declination increases
from 0 ◦ to 23 ◦ 26 ′ N.
In summer, theRA⊙ increases from 6 h to 12 h , its declination decreasing from 23 ◦ 26 ′ Nto0 ◦ .
In autumn,theRA⊙ increases from 12 h to 18 h , its declination changing from 0 ◦ to 23 ◦ 26 ′ S.
In winter, theRA⊙ increases from 18 h to 24 h , its declination changing from 23 ◦ 26 ′ Sto0 ◦ ,at
which time spring begins again.
In the southern hemisphere autumn begins when the Sun’s declination changes from south to
north, i.e. about March 21st. The seasons, autumn, winter, spring and summer succeed each other in
the same order in which they follow each other in the northern hemisphere.
As the Sun’s rate of increase in right ascension is not a constant, the seasons are of unequal length.
For any year, their lengths may be calculated from data tabulated in The Astronomical Almanac. One
approach is to determine by interpolation the times when the Sun’s declination is exactly zero (the
equinoxes) and when it achieves the maximum and minimum values. The latter are, however, difficult
to calculate exactly with accuracy using simple interpolation techniques.
The best and simplest method is to determine the time intervals between the apparent Sun passing
through the exact right ascension points of 0 h ,6 h ,12 h ,18 h and 24 h . This can be done by noting the
value of the right ascension at 00 h (DT) for the day the Sun passes through each of the previously noted
exact points. By also noting the right ascension values for the appropriate following day at 00 h ,the
times for the Sun being at the exact points can be obtained by simple linear interpolation. The time
intervals between each of the required exact points then provide values for the lengths of each season.