Vernal Equinox, the First Day of Spring

By Dr. Harold Williams

of Montgomery College Planetariumhttp://montgomerycollege.edu/Departments/planet/

12:57PM EDT March 20, 2014

Old Farmers Almanac Spring Equinox

S

N

W

E

Polaris

On the day of an equinox…

On the day of an winter solstice…

On the day of an summer solstice…

The Sky DomeSunrises and Sunsets for the Seasons

S

N

W

E

Polaris, NorthPole

Vernal or Autumnal Equinox,

EAST

SummerSolstice,

North of East

WinterSolstice,

South of East

Starting Points for the SunStarting Points for the Sun

Obliquity of the ecliptic• 23 ½ degrees, approximately, between day axis of

rotation and year axis of revolutionary orbit of the earth about the sun

• Causes of the Seasons• Seasons are caused by the tilt of the earths axis, 23

½ degrees, approximately.• Seasons are not caused by the fact that the earths

orbit is not a perfect circle, but the earth’s orbit, during the year, around the sun is slightly elliptical.

• Cause of the Inequality of the length of the Seasons

• The ellipticity of the earths orbit does cause the seasons to be slightly unequal in length.

Length of Seasons

• Winter 88 days 23 hours 54 minutes• Spring 92 days 18 hours 26 minutes• Summer 93 days 15 hours 29 minutes• Autumn 89 days 20 hours

Sun and Earth closest to each other, perihelionAround January 3 or 4Sun and Earth most distant from each other,

aphelion Around July 4

Equinoxes and Solstices

• The Vernal (Spring) Equinox (about March 21): The location where the Sun crosses the equator when going from south of the equator to north of the equator.– Position of the Sun: R.A. 0h, Dec 0o

• The Summer Solstice (about June 21): The location where the Sun is at its furthest north.– Position of the Sun: R.A. 6h, Dec +23.5o

Equinoxes and Solstices

• The Autumnal (Fall) Equinox (about September 21): Where the Sun crosses the equator when going from north to south.– Position of the Sun: R.A. 12h, Dec 0o

• The Winter Solstice (about December 21): The location where the Sun is at its furthest south.– Position of the Sun: R.A. 18h, Dec –23.5o

Some Resources on Seasons

• Seasons in the Wikipedia http://en.wikipedia.org/wiki/Seasons

• Spring Seasons in the Wikipedia http://en.wikipedia.org/wiki/Spring_%28season%29

• Tropical Year in the Wikipedia http://en.wikipedia.org/wiki/Tropical_year

People have known this for a long time.

Nowruz

Haft-Sin table

Jamshid ancient mythological figure of Greater Iranian culture and tradition.

Polar Vortex

• Jennifer Francis - Understanding the Jetstream Published on YouTube Feb 26, 2013

• http://www.youtube.com/watch?v=_nzwJg4Ebzo 5 minutes 31 seconds!

• Rossby Wave

• Vorticity and Vorticity Equations

Easter• The First Council of Nicaea (325)

established the date of Easter as the first Sunday after the full moon (the Paschal Full Moon) following the March equinox. Ecclesiastically, the equinox is reckoned to be on 21 March (even though the equinox occurs, astronomically speaking, on 20 March in most years), and the "full moon" is not necessarily the astronomically correct date.