U
The Physics
and Cultural
Significance
Spring in the
Northern
Hemisphere
Winter
in the
Northern
Hemisphere
Summer
in the
Northern
Hemisphere
Autumn in the
Northern
Hemisphere
of the
March
Equinox
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The earth’s seasons are determined by the tilt of
its rotational axis relative to the plane of the
Earth’s orbit around the Sun. Like a gyroscope,
the spin of the Earth serves to stabilize the
position of rotational axis in space – for this
reason, we find that the star, Polaris, above the
northern pole regardless of the time of year. For
about half a year, the northern hemisphere is tilted
towards the Sun. Then in the rest of the year, it is
tilted away, and the Suns’ rays are more oblique,
and the southern hemisphere gets more directly
illuminated. For this reason, the northern and
southern hemispheres always experience the
opposite season.
Besides the difference in how obliquely the Sun’s
rays illuminate the two hemispheres of the Earth,
a second consequence of this geometry is that the
hemisphere with the more oblique illumination
angle also suffers from shorter periods of sunlight.
The 24 hours of the day are divided such that the
nights are long and the daytime hours are short
.
Twice a year, there is a point during which neither
hemisphere has the advantage. At these times, the
Sun shines directly down onto the equator. For a
span of time near these dates the daytime and
nighttime time spans are nearly equal, giving rise
to the Latin term, “aequinoctium” which translates
to ‘equal night.’ From this, we get the word
equinox. In reality, sunrise/set is figured from
upper edge of the Sun and also factors in
atmospheric refraction, the bending of sunlight
near horizon. So overall, we get a little more
daylight than night at equinox.
Cultural Significance
In March, when the Sun is moving northwards the
equinox marks a time of transition and new
beginnings for many northern hemisphere
cultures, and is referred-to as the Spring or Vernal
equinox (it is the Fall equinox for the southern
hemisphere).
The equinox is commonly deemed to be the first
‘official’ day of Spring in western culture, but its
significance is more profound in other cultures.
New Year or Novruz
The Persian calendar begins each year at the
Vernal equinox as determined observationally at
Tehran. The ancient new year's festival of
Nowruz, or Novruz, can be celebrated March 20
or March 21 with up to two weeks of additional
festivities that are meant to retell the story of
creation and the ancient cosmology of Iranian and
Persian people.
As well as being a Zoroastrian holiday, it is also a
holy day for adherents of the Bahá'í Faith and the
Nizari Ismaili Muslims. The Bahá'í Naw-rúz is
calculated using astronomical tables - the new year
always starts at the sunset preceding the vernal
equinox calculated for Tehran.
The Vernal equinox is a holiday celebrated in
Azerbaijan, Afghanistan, Pakistan, Turkey,
Zanzibar, Albania, and various countries of
Central Asia, as well as among the Kurds.
Luni-Solar Calendars
The
Babylonian
calendar began with
the first full moon
after
the
vernal
equinox, the day after
the Sumerian goddess
Inanna's return from
the underworld (later
known as Ishtar).
The Indian National
Calendar starts the
year on the day after
the vernal equinox,
on March 22 (March
21 in leap years) with
a 30-day month (31 days in leap years), followed
by 5 months of 31 days and then by a further 6
months of 30 days.
The date of Easter varies from
year to year, but is
theoretically easy to predict
based on the relationship of
the Moon’s phases with the
equinox.
Gregorian calendar. The starting point in
determining the date of Orthodox Easter is
typically the actual date of the equinox according
to the Gregorian reckoning. This corresponds to
around April 3 in the Julian calendar. The Eastern
Church sets the date of Easter according to the
actual, astronomical, full moon as observed along
the meridian of Jerusalem.
Nominally, “the Sunday following the full Moon which
falls on or after the equinox will give the lawful Easter”
according to Bede in 725 AD.
The result of this combination of solar and lunar
discrepancies is divergence in the date of Easter as
observed by the different religions in most years.
Easter and the Equinox
However, this simple interpretation does not
reflect the actual ecclesiastical rules precisely.
One reason for this is that the full moon involved
is not an astronomical full moon. It is instead, the
14th day of a ‘calendar lunar month,’ also known
as the Paschal Full Moon. However, a lunar cycle
is actually 29.5 days. So the actual value for the
full moon is 14.75 days into the month, almost,
but not quite, a complete day later! As well, the
ecclesiastical date is fixed by convention to be
March 21, even though the actual equinox can fall
anywhere in the March 19-21 span.
Gregorian Easter can fall on 35 possible dates
between March 22 and April 25 inclusive. The
cycle of Easter dates repeats after exactly
5,700,000 years. April 19 is the most common
date, happening 220,400 times or 3.9%, compared
to an average of 3.3% for the rest of the days. The
extreme dates (March 22 and April 25) are much
less common.
There are other issues as well; the Eastern
Orthodox and Oriental Orthodox Churches
continue to use the Julian calendar which does not
correct for Leap Years as well as the newer
Spring
Astronomical
Gregorian
Julian
Jewish
Easter
Easter
Easter
Passover
27-Mar
27-Mar
1-May
23-Apr
16-Apr
16-Apr
11-Apr
1-Apr
8-Apr
31-Mar
21-Apr
28-Apr
20-Apr
Year
Full
Moon
2016
23-Mar
2017
11-Apr
16-Apr
2018
31-Mar
1-Apr
2019
21-Mar
24-Mar
2020
8-Apr
12-Apr
12-Apr
19-Apr
9-Apr
2021
28-Mar
4-Apr
4-Apr
2-May
28-Mar
Find out more
To learn more about the Royal Astronomical Society
of Canada, or membership in the Society, contact
your local Centre or the Society’s National Office:
Website:
www.rasc.ca
E-mail:
[email protected]
Telephone:
Phone: (416) 924-7973
Fax: (416) 924-2911
Mailing Address:
The Royal Astronomical
Society of Canada
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