Can anyone explain this? (Stellarium)

[furrysocks2]

I've just been playing with Stellarium.

Settings something like this:

• Show atmosphere - off
• Show planets - on
• Equatorial grid - on
• Ecliptic - on
• Cardinal points - off
• Constellation lines/labels - on
• Show ground - off
• Show fog - off

I had the Sun selected, centered on it, used equatorial mount, was and zoomed out to 150 degrees or thereabouts.

Fast forwarding so a month takes around 2 seconds, was cool. You could watch Mercury and Venus orbit the Sun, see the Sun move through the constellations of the zodiac, see the moon flash by every month, etc. Almost feels like the celestial sphere is a sphere, and you can pick out constellations, RA and dec, and generally get a bit dizzy.

 

Then I changed focus to the Moon...

 

Centered on the moon, and slowed down the fast-forward by one click, so that one day is approximately one second. What I see then is a pulsing, like a surge every second, as if speeding up and slowing down. If I reduce the FoV, zooming in on the moon a bit, it appears not to follow a straight line, but bounces around in time with the pulsing. This is easy to see with the ecliptic turned on.

What's going on?!

 

Very odd, until I understand what I'm looking at.... perhaps Stellarium is messing up, I don't know.

[Starlight 1]

Are you seeing that the Moon rise in the West and going back down in the West it so that right.

The Earth go backward and the Moon it locked to the Earth  so A new Moon come up in the West .

 

[Steve Ward]

It appears to pulse because the phases make it appear from invisible at 'new' to fully illuminated at 'full' and obviously all the different phases in between show different degrees of illumination ... i.e getting brighter then dimmer ... pulsing at that speed ... 

[furrysocks2]

1 hour ago, Starlight 1 said:

Are you seeing that the Moon rise in the West and going back down in the West it so that right.

The Earth go backward and the Moon it locked to the Earth  so A new Moon come up in the West .

 

I'm not sure if that's what I'm seeing - the moon's not rising as such...

 

1 minute ago, Steve Ward said:

It appears to pulse because the phases make it appear from invisible at 'new' to fully illuminated at 'full' and obviously all the different phases in between show different degrees of illumination ... i.e getting brighter then dimmer ... pulsing at that speed ... 

It's pulsing once a day though...

 

 

[furrysocks2]

Ahhh....

Could it be because Stellarium is showing a topocentric view (ie my observing position on the surface of the earth)? If it were geocentric (center of the earth) then all would appear stable, but because we spin on our axis, the motion of my observing position through space is uneven?

[Starlight 1]

I was thinking really be possible to see in real time having all the planets and the moon all moving, looking at the video Steve's explanation more plausible.

[laudropb]

I agree with Steve. It is due to the changing phases of the Moon.

[furrysocks2]

Just now, laudropb said:

I agree with Steve. It is due to the changing phases of the Moon.

But the wobble period is 1/day - so I don't understand why phase comes into it as you can see the phase in the animation wax slowly from new to crescent...??

[furrysocks2]

My hypothesis is still that the wobble is due to parallax - I can't get my head around an argument for phase.

 

I took ra/dec values from Stellarium on the hour, every hour, for a day, and for the two midnight values linearly interpolated reference sets. Graphing the difference between the two gave me these:

 

Converting RA variation in hours to degrees and summing with DEC, I get this:

 

 

Calculating my approximate displacement over half a rotation of the earth using c=2Rsin(a/2) at latitude 56.25N, I get  approximately 7080km - this used an arbitrary value for Earth's radius and does not account for it being a spheroid.

Using trigonometry to calculate the length of the hypotenuse of a right angled triangle using half my displacement and half the variation in angle, I get a value of 414,730 km.

In Stellarium, the values were taken for 19th Jan, 3 days from apogee on 22th Jan, for which an arbitrary online resource gives me a value of 404,914 km, and another site gave me approx 402,500km on 20th Jan. So if I ignore the radius of the two bodies and whether the quoted or calculated measurements include these or not, and considered the distance to the moon as 402,000km, I would have an error of 3.17%, which could easily be mistaken for a successful calculation.

 

Did I do it, or a meringue??

 

Edit: if, rather than summing RA/DEC variations, I combine them with Pythagoras, I get 400,401km, error of -0.4%. Not sure I know exactly what I'm doing, though.