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Caption: The
ecliptic,
equinoxes and
solstices,
and
seasons
in the celestial sphere perspective.
Features:
- The ecliptic is the path of
the Sun on the
celestial sphere during
a solar year.
The Sun moves
eastward---well to
1st-order---there
is an angle discussed below.
This motion is superimposed on the daily
westward of the
celestial sphere
on the celestial axis.
- The angle 23.4° of the
ecliptic from the
celestial equator
is an obvious consequence of the
Earth's axial tilt 23.4°
from the ecliptic axis
(i.e., the axis perpendicular
to the Earth's orbital plane
which is the ecliptic plane).
- There are 4
special points
on the ecliptic:
2
equinoxes and
2
solstices:
- The 2
equinoxes are where
the ecliptic crosses
the celestial equator.
The vernal equinox (c.Mar21)
is the point
where the Sun
crosses the ecliptic
going north
and the fall equinox (c.Sep21)
is the point
where the Sun
crosses the ecliptic
going south.
- The 2
solstices
are where the ecliptic reaches
an extreme point in declination.
The summer solstice (c.Jun21)
is the highest point in declination
and the
winter solstice (c.Dec21)
is the lowest point in declination.
- Note that terms solstice
and equinox both have two meanings:
- The point on the
celestial sphere.
- The event of the
Sun being at that point.
The event marks the beginning of a
season:
1) the vernal equinox (c.Mar21)
begins spring;
2) the summer solstice (c.Jun21)
begins summer;
3) the fall equinox (c.Sep21)
begins fall;
4) the winter solstice (c.Dec21)
begins winter.
Usually only context tells you which meaning is meant.
- Now judging from the heating effect of the
Sun, one would at
first glance think that the astronomical start dates for the
seasons
would be about the
middle of the climatic seasons.
However,
seasonal lag
(mainly due the time it takes
large bodies of water
to heat and cool)
which is of order a month
makes the astronomical start dates reasonably appropriate for the
start dates of the climatic seasons.
- The approximate
calendar dates given for the
equinoxes and
two solstices
can also be regarded as their
fiducial
calendar dates.
The variation in
the calendar dates
is between day 20 and 23 of the respective month
is partially due to the cycle of
common years (365 days) and
leap years (366 days).
Since common year
is shorter than
the solar year = 365.2421897 days (J2000),
a run of common years
moves the
equinox
and solstice
calendar dates to later
and then a
leap year
(which is longer than a
solar year)
overcorrects.
Other astronomical effects cause the variations in the start dates to be more
than 1
metric day
=24 h = 86400 s.
In any case, the average month day
is about day 21, except for the
fall equinox (c.Sep21)
where its more like day 22 it seems---but that is hard to remember, and so
yours truly
sticks with day 21 as the fiducial day.
- The vernal equinox
as a point
on the celestial sphere
is also the zero point of
right ascension (RA)
(i.e., 0 h RA)
in the
equatorial coordinate system.
One can distingish this meaning of
vernal equinox
when one wants to as
vernal equinox (0 h RA)
(AKA First Point of Aries).
Credit/Permission: ©
David Jeffery,
2003 / Own work.
Image link: Itself.
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File: Celestial sphere:
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