KEPLER'S PARALLAX OF MARS
-----------------------
John Pazmino
NYSkies AStronomy Inc
nyskies@nyskies.org
www.nyskies.org
2011 December 17
Introduction
----------
The NYSkies Astronomy Seminar of 16 December 2011 discussed
methods of determining the distances to celestial objects. One was the
parallax or triangulation method. The technique involves measuring a star
from different places in the Earth's orbit. The triangle of Earth at
time 1, Earth at time 2, and star is solved for the Earth (either
one)-star leg. The length of this side is enormous compared to the
Earth's orbit, in the order at the very least of 100,000 Earth orbit
radii.
We mentioned that Kepler tried triangulation on the planets to
discover that the planet orbits are ellipses, not off-centered
circles. The question came up that as Earth moves from place 1 to 2 in
her own orbit, the planet also moves. The two sightlines can not make
a closed triangle. A star, while it has proper motion, is essentially
stationary in space for the decade or so it takes to get sufficient
observations for a good parallax.
How did Kepler get around the problem of the moving planet?
Kepler's work
-----------
Kepler flourished in the early 1600s, a contemporary of Galileo
and a worker at Tycho's observatory. When he took up the problem of
planet motions he realized that it was futile to find parallax based
on the Earth's own diameter. Tycho tried that for the supernova and
comet of the 1570s and found them to be well beyond the Moon's
distance.
Tycho could measure no parallax on the planets, they, too, being
indefinitely remote. At least he proved that both comet and supernova
(stella nova) were in the celestial realm, not some luminous
apparition among the clouds or under the Moon.
Kepler from his work with Tycho could mine observations of the
planets to find clues to the planetary motions. He also had faith that
Tycho was a careful and cautious observer whose data were of the
highest precision up to his era. He was an adhaerent of the Copernicus
solar system in which Earth orbits the Sun. He designed to try
parallax based on the diameter of the Earth's orbit, which is
immensely larger than the very Earth herself.
Kepler did not know the length of the orbit radius, in kilometers
or what ever units were used in his time. He set the Earth orbit
radius to unity as a supersize meterstick. We still do this today,
calling this length the astronomical unit or AU. Since the Earth
varies her distance from the Sun during the year, the radius is the
mean distance, about 149,600,000 kilometers.
Clever trick
----------
For the stars, Kepler recognized that they were so very far away
that they were essentially fixed in space, In fact, Kepler engaged the
stars as the 'ground' against which to lay out the planet orbits.
His zeroth law, commonly cited as part of the first law, stated
that the orbits of planets are planes fixed in space and passing thru
the Sun. This is a truly profound statement. For the first time in
human history Earth was replaced as the foundation of motion and
motions were banked against the enclosing sphere of fixed stars.
His effort to find a frame or platform for citing absolute motion
eventually became the basis for Einstein's theory of relativity quite
300 years later. The separation of the two concepts, planes and
ellipses, usually rolled into the first law brings out this feature of
Kepler's thinking.
He also knew that planet orbits, under Copernicus, were stable and
rigid in their planes. The planets traced the same path over and over
again during each round of the Sun.
To get a sight on Mars from different places in Earth's orbit,
Kepler had to be sure that Mars was in the same place in his own orbit
at each sighting. He did this by picking Tycho's records of Mars
spaced one Mars year apart. After one round of the Sun Mars stands at
the same place in his orbit, ready for a new sighting from Earth.
The practical difficulty was that Tycho, while he made routine
measures of Mars, did not purposely keep account of the period or year
of Mars. Kepler had to do with records close to yearly intervals and
interpolate among them.
With pairs of such observations in hand, Kepler plotted the path
of Mars around the Sun and fitted a curve for it. The curve of best
fit ended up being an ellipse and not a circle displaced one way or
another from the Sun. The rest of the story is history.
Replicating Kepler's method
-------------------------
Here we use a modern computer ephemeris or planetarium program to
replicate Kepler's work with Mars. By now the 2-thous virtually all
astronomy programs generate competent positions and motions of the
planets. It really doesn't matter which you use. As long as it gives
readouts of various parameters about the planet when selected from a
chart of table, the program is good for this exercise.
The figures here are my own taken for the date of the Seminar, 16
December 2011, and dates one and two Mars years before then. After
working thru Kepler's method for these fates you may try for any other
set of dates at one Mars year intervals.
The period of Mars's orbit was well known from both the Ptolemaeus
and Copernicus models of the world, In the Copernicus scheme the orbit
circulated around the Sun. In the Ptolemaeus system the orbit was the
deferent around Earth.
The modern value is 1.0807 Earth year or 686.9 Earth day. This
figure may differ slightly among references because an orbit is not a
trolley track in space. It wiggles and wanders from gravity tugs of
other planets.
Earth's orbit
-----------
Some treatments of Kepler's planetary work state that Kepler
assumed Earth had a circular centered orbit. This makes the maths
easier but there was no reason to make this assumption. The Earth's
orbit in Copernicus was well established with varying distance from
the Sun during the year. Kepler used the instant distance for each of
Tycho's records.
Tycho, by the way, favored a mixed Copernicus-Ptolemaeus world
where the planets orbited the Sun but the Sun then orbited Earth. For
the predicting the places and movement of the planets in the sky this
Tycho system was just as good as the other two. Kepler went with the
pure Copernicus model.
Maths
---
Kepler was blessed by a fantastic historical accident. In 1603-
1604 logarithms were invented! Both the common series by Briggs and
the natural one by Napier were published almost simultaneously.
Logarithms made calculations immensely easier by reducing operations
by one level. Multiply-divide were handled by addition-subtraction;
power-root, multiply-divide.
Logarithm tables were published for the trig functions, making
them as easy to manipulate as regular numbers. Kepler himself noted
that if it wasn't for logarithms, he probably would have given up with
his planetary studies.
Until the introduction of electronic calculettes in the 1970s ws a
astronomers had to be fluent in logarithmic computations. Many
astronomy equations were written from start in logarithms. The
calculette by 1980 become so pervasive and cheap that school began
dropping tuition in logarithms. Probably now in the 2-thous,
logarithms are a quaint footnote in school maths books.
For this article you should have a scientific or engineering
calculette, not arithmetic one. Excellent models are at sale for a
couple tens of dollars that will satisfy every astronomy situation.
Triangulation
-----------
Parallax works with a triangle set up on a baseline with apex at
the target body. Sightlines to the target are made at each end of the
baseline with appropriate angles taken. By trig formulae the length of
the sightlines are worked up for the distance to the target.
The work here employs the ecliptic coordinates, these being vastly
simpler to describe the motion and position of planets than the
equatorial right ascension and declination. To further ease the maths,
I neglect the latitude component and let Mars run in the ecliptic in
pure longitudinal motion. This does let small errors creep in but the
principles are just as valid as with both component factored in.
Observing Mars
------------
From an ephemeris program we collect the following 'observations'
of Mars spaced one Mars year apart.
-------------------------------------------------------
parameter | 16 Dec 2011 | 28 Jan 2010 | 12 Mar 2008
--------------+-------------+-------------+------------
long of Mars | 165.3 deg | 130.4 deg | 92.6 deg
long of Earth | 83.9 deg | 128.2 deg | 172.0 deg
dist of Earth | 0.984 AU | 0.985 AU | 0.994 AU
-------------------------------------------------------
for pair 2011 Dec 16-2010 Jan 28
---------------------------------
angle at Sun | abs(83.6 deg-128.2 deg) = 44.3 deg
--------------------------------------------------
Your peculiar program may offer the longitude of Sun, not Earth,
if it confines to only an Earth viewpoint. The longitude of Earth as
seen from Sun is merely 180 degrees added or subtracted from the solar
longitude. Do which ever throws the value within 0-360 degrees.
For added realism, you can pick off the longitudes of Sun and Mars
from a computer planetarium by scaling off of the background stars and
the ecliptic coordinate grid. The accuracy will deteriorate somewhat
from that of a numerical output of an ephemeris program but the
principles are just as valid.
Sun-Earth triangle
----------------
We take the three pairs of observations to get sightlines to Mars:
2008-2010, 2010-2011, and 2088-2011. The details are here laid out for
the 2010-2011 pair and you can use them as the model to work out the
other two pairs.
We need the baseline length and orientation for the 2010 and 2011
dates. The sketch below shows the geometry.
/ E11
/ |
/ |
E10 | 83.9d
\ | 0.984AU
128.2d\ |
0.985AU \ |
S
You really better make sketches of the planetary situation to keep
the angles and distances right way round. This is specially true when
working with trigonometric formulae. Pass up on this procedure will,
WILL, throw your results wildly wrong.
The baseline is E11 to E10 and is found by the Law of Cosines We
first need the angle at the Sun, which is the difference in the Earth
longitude for the two dates, 128.2 - 83.6 = 44.3 deg. The absolute
value is used, the signum being discarded.
c^2 = a^2 + b^2 - (2*a*b*cos C)
(E11_E10)^2 = (S_E11)^2+(S_E10)^2-2*(S_E11)*(S_E10)*(cos S)
= (0.984)^2+(0.985^2)-(2*0.984*0.985*cos 44.3)
= 0.551 AU2
(E11_E10) = 0.742 AU
One common mistake is to miss taking the square root of the c^2!
You'll end up with way too short a distance to Mars, one that may
escape notice unless you're familiar with Mars apparitions.
The angles at E11 and E10 are the same by the symmetry of the
Earth orbit and are (180-44.3)/2 = 67.8d. This ia not exactly true but
since the two legs from Sun to Earth are so nearly the same, the error
is very small.
The results so far are gathered here for easy reference
-------------------------
E11_Sun_E10 triangle
--------------------
E11_E10 | 0.742AU
S angle | 44.3 deg
E11 angle | 67.8 deg
E10 angle | 67.8 deg
--------------------
Mars-Earth triangle
-----------------
We now need the base and angles for the Earth-Mars triangle. The
baseline length we already got from above as 0.742 AU. The two angles
at the ends of the baseline are found by studying the sketch below.
165.3d
M- - - - [98.6d]
\ - - - - E11
\ / |
\ / |83.9d
130.4d\ / |
\ / 0.742AU|
\ /
E10
[182.2d] \
\128.2d
\
The angles between Sun-Earth-Mars at the two dates are the
differences between the Sun and Mars longitude, rectified to the
proper quadrant. I mark them in bumpers in the diagram. They are
nearly the elongations of Mars from Sun in the Earth's sky.
Here is where a sketch saves your life. The orientation of the
lines in the figure are based on ecliptic longitude. By adding or
subtracting them against 180 or 360 degrees you obtain the large
interior angles. You must pay attention to the sense of the angles!
We need the angles along the baseline, so we subtract out the 67.8
deg from each of the elongation angles. At E10 we have 182.2d-67.8d =
114.4d and at E11, 98.6d-67.8d = 30.8d. The third angle, at Mars, is
180d-(114.4d+30.8d) = 34.9d. Here are the new parameters
-------------------
Earth-Mars triangle
-------------------
dist E08_E10 | 0.742 AU
angle at E10 | 114.4 deg
angle at E11 | 30.8 deg
angle at Mars | 34.9 day
-------------------------
With all three angles and one side, we employ the Law of Sines to
get the length of each Earth-Mars sightline.
a/sin A = b/sin B = c/sin C
(E11_M)/(sin E10) = (E10_M)/sin E11) = (E11_E10)/sin M
(E11_M) = (E11_E10)*(sin E10)/(sin M)
= (0.742AU)*(sin 114.4d)/(sin 34.9d)
= 1.181 AU
This compares favorably with the computer ephemeris value of 1.183 AU
for 16 December 2011. Kepler had no such modern planetary tools. Doing
the same calc for the side E10_M yields 0.662 AU, also in good
agreement with an ephemeris.
Note well that by picking dates when Mars returns to the same
place in his own orbit we pinned Mars in space relative to Earth by a
triangulation just as if Mars was standing still.
This process Kepler repeated for many pairs of observations spaced
at multiples of a Mars year to build up a plot of Mars around the Sun.
Distance from Sun
---------------
The distance of Mars from Sun comes directa mente from the
triangle Sun-Earth-Mars for either date. We have for the 2011 case the
diagram below
M\\\\\\\\ 1.181AU
\ \\\\\\\E11 [98.6d]
\ |
\ |
\ | 0.984AU
\ |
\ |
\|
S
We apply the Law od Cosines having two sides and the included
angle. The included angle is almost the elongation of Mars from Sun,
98.6d in bumpers in the diagram.
c^2 = a^2 + b^2 - (2*a*b*cos C)
(S_M)^2 = (E11_S)^2+(E11_M)^2-(2*(E11_S)*(E11_M)*cos E11
= (0.984)^2+(1.181(^2-(2*(0.984)*(1.181)*cos(98.6))
= 2.711 AU2
(S_M) = 1.646 AU
Doing this with the 2010 date and parameters also yields 1.646 AU.
The two distances apply to the same Sun-Mars line and should be very
nearly equal. An ephemeris gives 1.647 AU for Mars on both dates.
Graphical solution
----------------
If the maths are out of reach, you can work the problem entirely
by graphical methods. You must have steady hand and the proper tools.
Have a T-square, drawing board, protractor, triangles and ruler, and
sharp pencils.
Be diligent in your linework, taking care to keep the lines crisp.
Home the protractor or ruler on the diagram accurately and read the
scale to the tenth degree if feasible. For a small protractor of only
a few centimeter diameter you'll have to make do with readings of the
quarter degree at best.
The results will be a bit rough but the concepts are fully
demonstrated.
An other trick
------------
Kepler wanted other places where Mars is known to occupy at each
lap of the orbit. He used the two nodal crossings, ascending and
descending. He verified the period of Mars by noting the interval
between successive ascending or descending node passes. He then
captured the ecliptic longitude of Mars at each pass to construct more
triangnlations.
If you try this trick you hunt for dates when Mars crosses the
ecliptic. This is easiest done with an ephemeris giving ecliptic
coordinates. The node pass occurs when the latitude of Mars passes
zero degrees.
Working with angles
-----------------
In this simple example of triangulation we came across some
troubles with angles. That's why I stress that you must sketch out the
geometry to catch silly, or stupid, blunders. It's in the nature of
angles that the troubles come from and there are two special ones to e
on watch for.
First is the walk between angle and trig function. Each angle has
its own unique value for the trig functions. On a calculette you can
key in the angle and get back the proper one and only function value.
It doesn't work so well from function back to angle! Every value
for a trig function has TWO angles, not just one. In general the
calculette gives you the smaller or positive one of the two possible
angles. You must thru supplemental considerations decide which of the
two is the proper result. That's what the sketch is for.
The other issue is that the angle so returned may be in any of the
four quadrants of 0-90 deg, 90-180 deg, 180-270 deg, 270-360 deg. You
have to inspect the geometry carefully to get the proper quadrant for
the returned angle.
I warn that this chore is an integral part of astronomy, a
discipline that lives off of angles.
Conclusion
--------
This is an exercise for a high school class versed in trigonometry
or a freshman college class. It shows how it is possible to fathom the
solar system by exploiting the stability of planet orbits. It also
shows the cautions in manipulating angular measures and trig
functions.
The simulated observations from a planetarium program adds some
realism by taking measurements off of the sky on the computer screen.