Tropical Events: the solstices and equinoxes

 Tropical Events: the solstices and equinoxes

This presentation is a simple laymans study of some interesting aspects of Earths orbital history, especially concerning solstices and equinoxes, and the changing lengths of the days, seasons, and years. The charts below are from an Excel spreadsheet, created with an open source Excel Add-In, both of which I am making available at the end of this webpage: as I would have loved to have started out with something similar 10 years ago, when I first became interested in "the motions of the heavens" and the workings of the solar system.

My first inquiry concerned the "mean tropical year": why does it change?

chart 21

days per mean tropical year

in dynamical days of 86400 SI seconds

days per mty

mty.png

365.242750 365.242625 365.242500 365.242375 365.242250 365.242125 365.242000 365.241875 365.241750 365.241625 365.241500

days

-100,000 -80,000 -60,000 -40,000 -20,000 0 20,000 40,000 60,000 80,000 100,000

calendar year

It is mostly due to the changing speed of the precession!

The rate of precession is shown below in arcseconds per Julian year (365.25 days), a standard time increment used by astronomers.

chart 2

rate of precession

arcseconds per Julian year degree 7 polynomials from data La93(0,1)

quasi-periodic approximation, Laskar et al 1993 Laskar 1986 (?10T)

precess.png

53.00 52.75 52.50 52.25 52.00 51.75 51.50 51.25 51.00 50.75 50.50 50.25 50.00 49.75 49.50 49.25 49.00 48.75 48.50 48.25 48.00

arcseconds

-100,000 -80,000 -60,000 -40,000 -20,000 0 20,000 40,000 60,000 80,000 100,000

calendar year

Comparison with the next chart shows us the relationship of the precession rate to the obliquity of the ecliptic (the tilt of our spin axis in relation to our orbital plane):

chart 4

obliquity

quasi-periodic approximation, Laskar et al 1993 degree 7 polynomials from data La93(0,1)

obliq.png

24.7500

24.5000

24.2500

24.0000

23.7500

23.5000

degrees

23.2500

23.0000

22.7500

22.5000

22.2500

22.0000

-100,000 -80,000 -60,000 -40,000 -20,000 0 20,000 40,000 60,000 80,000 100,000

calendar year

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The precession and the obliquity are from Jacques Laskar (et al 1993), an awesome astronomer at the Bureau des Longitudes in France who applies the concepts of Newtons equations of motion to the entire solar system for millions and millions of years! He then approximates his results with one single equation (called a quasi-periodic approximation) that is very close to the "true solution", at least for a few millions of years. This one for the precession and obliquity gives good results for 18 million years in the past, according to his solution usually designated La93(0,1), and it also gives good approximate results for 2 million years into the future. The Add-In can calculate with either the quasi-periodic approximation formulae, or from 7th-order polynomial expressions that interpolate the data from his La93(0,1) solution. The spreadsheet used to create these charts also uses the formulae from Laskar 1986 (valid for ?10,000 years from J2000), providing a comparison during the time period "closer to home".

My next question concerned the lengths of the seasons: "why are they changing?"

- 3 -

-30,000 -25,000 -20,000 -15,000 -10,000

-5,000 0

5,000 10,000 15,000 20,000 25,000 30,000

chart 1

Earth eccentricity

eccentricity degree 7 polynomials from data La93(0,1)

quasi-periodic approximation (Laskar et al, 2004) Laskar 1986 (?10 T)

calendar year

ecc1.png

0.0210 0.0190 0.0170 0.0150 0.0130 0.0110 0.0090 0.0070 0.0050 0.0030 0.0010

chart 19

Lengths of the Seasons (TD)

(northern hemisphere) in days of 86400 SI seconds

Winter

Spring

Summer

Fall

seasons1.png

95 94 93 92 91 90 89 88 87

days per season

-30,000 -25,000 -20,000 -15,000 -10,000

-5,000 0

5,000 10,000 15,000 20,000 25,000 30,000

calendar year

It is mostly due to the changing eccentricity and motion of the perihelion!

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As the eccentricity approaches zero (circular orbit), the seasons become nearly equal.You can detect the motion of the perihelion from the previous chart, as it moves from season to season, causing that season to be "shortest". The next chart shows the speed of the perihelions motion as the length of the anomalistic year:

chart 6

anomalistic year (TD)

days per anomalistic year degree 7 polynomials from data La93(0,1)

quasi-periodic approximation (Laskar et al, 2004) Laskar 1986 (?10 T)

anom_yr.png

365.277636 365.275636 365.273636 365.271636 365.269636 365.267636 365.265636 365.263636 365.261636 365.259636 365.257636 365.255636

days

-30,000 -25,000 -20,000 -15,000 -10,000

-5,000 0

5,000 10,000 15,000 20,000 25,000 30,000

calendar year

Laskar (et al 2004) has developed another approximation formula for both the eccentricity and perihelion, valid from -15M (million years ago) to +5M (million years in the future). Again the Add-In has retained good accuracy by interpolating Laskars 1993 data with polynomial expressions. The method used is on p.32 in the book "Astronomical Algorithms" by Jean Meeus (1998), referred to as "Lagranges interpolation formula".

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