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I have noticed that the tilt of the Sun is near zero currently and the Earth is at Perihelion. Is this just coincidence or is there a connection?
Hi Dave,
The position of the Earth’s point of perihelion is fairly random.
Over astronomical timescales, the point of perihelion undergoes considerable precession over hundreds of millions of years, and so there’s not much significance to where it happens to be right now. Further evidence that perihelion points are random comes from the fact that the perihelion points of the other planets are all in different positions around their orbits (see the attached screenshot, taken from https://in-the-sky.org/solarsystem.php , showing the terrestrial planets, with perihelions marked ‘P’).
The fact that the sun’s tilt is always reasonably close to zero is not such a coincidence – the Sun and planets formed out of a common protostellar nebula, and both the Sun and the planetary orbits take their plane of rotation from the rotation of the gas cloud from which they formed.
Cheers,
Dominic
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Thank you Dominic. I am not sure that I was very clear. Each time I image the Sun, I use Tilting Sun software to indicate how much the Sun appears tilted from the Earth’s point of view. A day or two ago the Sun went from tilting to the left through zero and is now very slightly tilting to the right. It was at perihelion on January 3 when the apparent tilt of the Sun was zero. I wondered if that was coincidence or not.
Dave
Short answer, as Dominic says this is a coincidence. Longer answer as follows…
I assume that the tilt you mention is the apparent position angle of the sun’s rotation axis in equatorial coordinates. This is a function of the sun’s axial tilt with respect to the ecliptic (around 7.3 degrees), the Earth’s axial tilt (around 23.5 degrees) the orientation of the two spin axes in space and where we are in our annual orbit around the Sun. In the short term the rotation axes of the Sun and Earth and the plane of the Earth’s orbit have a fixed orientation so as we travel around the Sun the relative geometry changes on an annual cycle.
Our view of this tilt depends on the coordinate system we use.
In ecliptic coordinates (i.e. not including the Earth’s axial tilt) you would see the rotation axis nodding from side to side in a simple sinewave over 12 months going through a minimum in March when we see the south pole tilted towards us and in September when we see the north pole. In between it reaches its maximum tilt of 7.3 degrees tilted left or right. When you look in the equatorial coordinate system you have to add in the effects due to the Earth’s axial tilt as well. This makes the geometry more complex since you have two sinewaves adding together but, as you have noted, the tilt is around 0 in early January and reaches around 26 degrees in early April.
The orientation of the Earth’s perihelion in its orbit is completely unrelated to the orientation of the rotation axes and, as Dominic noted, precession affects the Earth’s axis orientation and the perihelion orientation over the long term so this “coincidence” will not last forever.
The thing that is surprising, following up on Dominic’s comment, is why there is a such a large angle between the Sun’s rotation axis and the normal to the Solar System’s invariable plane (essentially the perpendicular to Jupiter’s orbit plane). Since the Sun and its planetary system formed out of the same rotating cloud of material you would expect them to have the same angular momentum orientation. The fact that the planets now move in a plane tilted at around 6 degrees to the Sun’s equator is a bit surprising. There are some quite inventive theories out there to explain this.
This is a good summary of the angular momentum misalignment and some of the theories put forward to explain it:
https://iopscience.iop.org/article/10.3847/0004-6256/152/5/126
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