science: summer solstice 2008

happy summer solstice!!!!

this year's summer solstice occurred on 20 June at 23:59 UTC - that is 21 June at 05:29 local mean time
earth is furthest to the sun today at 152,029 km according nasa

for locations on the tropic of cancer, the sun appears to be directly overhead at noon
for locations on the arctic circle, the sun will set at midnight
for locations above the arctic circle, the sun will not set

the following article explains the science of summer solstice

tilted to its max
earth is furthest to the sun
and i missed it!!

today of all days
for someone in the arctic
the sun does not set

for us sri lankans
today is as yesterday
just further to sun

hmmm i hope you liked my haiku...

earth's axial tilt

earth has an axial tilt of about 23.44°
that is.. the angle between earth's axis of rotation and earth's ecliptic is ~23.44°

Wikipedia: "The ecliptic is the apparent path that the Sun traces out along the sky, in relation to the stars, and aligns with the planets throughout the course of the year."

the ecliptic plane is earth's mean orbital plane about the sun... the ecliptic is plane intersecting the ecliptic plane and the celestial sphere... the celestial sphere is an imaginary rotating sphere with earth's center as its center and earth's rotation axis as its rotation axis...

so basically its the sky viewed from the center of the earth... and all the objects in the sky can be placed in this sphere...

declination angle, δ

declination angle is the angle measured above or bellow the celestial equator. for a given day, since the orbital movement around the sun is very small (sun, is given by:

δ_sun = 23.44 * sin((360*(284 + N)/365)) -----(1)

where N is the day number (eg. N=24 for Jan 24)

the above model is an empirical formula obtained using curve fitting.

there is also a fourier representation of this:

δ_sun = 0.006918 - 0.399912 cosγ + 0.070257 sinγ - 0.006758 cos 2γ + 0.000907 sin 2γ - 0.002697 cos 3γ + 0.00148 sin 3γ -----(2)

where γ = (2π/365)(N-1)

lets get back to equation (1)
it is important to appreciate that it is a sine function. thus δ_sun varies from +23.44° to -23.44°.

there will be:

1. two instances when δ_sun = 0, know as vernal equinox
2. and autumnal equinox
3. an instance when δ_sun = +23.44°, known as summer solstice
4. and an instance when δ_sun = -23.44°, known as winter solstice

on summer solstice...

• the earth is furthest to the sun² at 152,029 km
• for locations on the tropic of cancer, the sun appears to be directly overhead at noon
• for locations on the arctic circle, the sun will set at midnight
• for locations above the arctic circle, the sun will not set

so whats in it for us sri lankans...

nothing i guess hehe :)
its just another hot and humid day!!

references

1. Earth's Seasons, Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020 from the United States Naval Observatory's Astronomical Applications Department
2. NASA - JPL Solar System Simulator
3. NASA: Solar Radiation and the Earth System
4. Wikipedia: Solstice
5. kanabona.com: Winter Solstice 2007
6. kanabona.com: Winter Solstice 2007