Orbital Mechanics: Earth's position

[spacester]

I am trying to create, among other things, an orrery that accurately depicts Earth and Mars in their true elliptical orbits. I have learned how to calculate flight paths from Earth to Mars, which of course depends on their relative angular positions and true distances from Sol. This is a pointless exercise to me if it does not reflect reality.

No matter how much I try, I am unable to convince myself that what I see here is correct:

https://ssd.jpl.nasa.gov/?ss_inner

It is almost inconceivable to me that JPL would be wrong, but it just does not add up. My calculations work if I make a correction to what this diagram and some others tell me.

First, if viewing "from above", multiple sources elsewhere (but not the caption here) tell me the planets rotate CCW.

Secondly, the caption says that the vernal equinox for Earth - the First Point of Aries - is to the right.

The diagram shows the positions as of Jan 1 2018, so Earth is shown just a few days from Perihelion on Jan 4. Note that this places the perihelion another 3 degrees CCW, so clearly more than 90 degrees from the Vernal Equinox. But the number of days from Jan 4 to March 20 is 27 + 28 + 20 = 75 days, which is clearly less than 90 degrees of revolution for Earth in its orbit. (75/365)*360 = 73.97 degrees.

So on the face of it, something on that diagram is wrong. It appears to me that if the Vernal equinox is to the LEFT, then it all makes sense. This seems to be confirmed by this simulator:
http://www.astronoo.com/en/articles/positions-of-the-planets.html
(Zoom in and run the clock backwards to Jan 1 2018)

I would not be posting here if I hadn't already beat my head against this discrepency extensively. I want to get this right. JPL is wrong?

FWIW I found exactly one obscure source that states that the Argument of Perihelion as applied to Earth actually gives the position *opposite* (IOW 180 deg) to the planetary position. Seriously?

For the other planets, the orbital position is fairly straightforward: Locate the Longitude of the Ascending Node relative to the First Point of Aries. From that node, locate the perihelion according to the Longitude of Perihelion, which is also referred to as the Argument of Perihelion (or possibly they are different?). With the perihelion established, the orbital ellipse can be oriented in our reference coordinates of the plane of the ecliptic and the First Point of Aries. The position of the planet on that ellipse for a given date can then be found from the True Anomaly as given by JPL's Horizons database.

Earth's orbit defines the plane of the ecliptic, so technically there is no ascending or descending node. So, unlike the other planets, there technically should be no such thing as the Longitude of Perihelion. Yet the data here:
https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
does list that parameter. Aha, I thought, there is an -11.26064 degree adjustment to possibly consider. But the Longitude of Perihelion = 102.94719  and if I subtract the 11.26 from that, I get 91.68, which is still not consistent with 73.97 from Perihelion to Equinox.

The only thing I am absolutely sure of is when the Equinox and Perihelion occur. It has to be 74 degrees plus/minus a couple degrees. But the JPL diagram clearly shows otherwise. (BTW I will be ignoring the difference in inclination of the Martian orbit for flight path calculation.)

Have I really found an error on an authoritative JPL webpage? As I said, if I place the First point of Aries to the LEFT instead of to the RIGHT as the caption states, everything works. I think.

(I know that the First Point of Aries has drifted from Aries to IIRC Pisces since antiquity, but if each epoch is based on the Vernal Equinox, and if only positions within the solar system are considered that should not matter.)

Any insight would be helpful, TIA.

[Jorge]

I am trying to create, among other things, an orrery that accurately depicts Earth and Mars in their true elliptical orbits. I have learned how to calculate flight paths from Earth to Mars, which of course depends on their relative angular positions and true distances from Sol. This is a pointless exercise to me if it does not reflect reality.

No matter how much I try, I am unable to convince myself that what I see here is correct:

https://ssd.jpl.nasa.gov/?ss_inner

It is almost inconceivable to me that JPL would be wrong, but it just does not add up. My calculations work if I make a correction to what this diagram and some others tell me.

First, if viewing "from above", multiple sources elsewhere (but not the caption here) tell me the planets rotate CCW.

Secondly, the caption says that the vernal equinox for Earth - the First Point of Aries - is to the right.

The diagram shows the positions as of Jan 1 2018, so Earth is shown just a few days from Perihelion on Jan 4. Note that this places the perihelion another 3 degrees CCW, so clearly more than 90 degrees from the Vernal Equinox. But the number of days from Jan 4 to March 20 is 27 + 28 + 20 = 75 days, which is clearly less than 90 degrees of revolution for Earth in its orbit. (75/365)*360 = 73.97 degrees.

Why are you confusing the issue by bringing perihelion into it at all? Just go straight from the time of the diagram to the equinox. Jan 1 to Mar 20 is 31 + 28 + 20 = 79 days, which is also clearly less than 90 degrees of revolution. (79/365)*360 = ~78 degrees. So the diagram shows Earth less than 90 degrees CCW from the equinox. Remember that at the vernal equinox, the sun will be in the +X direction from Earth (conversely, the Earth will be on the sun's -X axis, i.e. to the left of the sun on the diagram).

[spacester]

Remember that at the vernal equinox, the sun will be in the +X direction from Earth (conversely, the Earth will be on the sun's -X axis, i.e. to the left of the sun on the diagram).

Ding Ding Ding that is a winner! Thank you! I expected a long wait and much confusion and discussion. I came to the right place.

The First Point of Aries is NOT the position of Earth, it is the position of the Sun! Duh! That clears up everything, well answered!

I do need to worry about the periapse in order to orient the orbit and thus to know the true anomaly = 0 position.

The diagram is not my primary reference, it was just the easiest way to illustrate my question. I am using emphemerides for precice calculations, drawing this on CAD and implementing a general solution that fully accounts for eccentric orbits.

I knew JPL could not be wrong about such a thing.