Beginner question about planets.

[Jnicholes]

Hi everyone,

I have not been on for a while, a lot has been going on here. Haven’t had much of a chance to do astrophotography in a while.

I have a simple beginners question that I felt like posting.

First, some context. On September 20, 2022, at 9:14 PM MST, AND on October 2, 2022, 8:51 PM MST, I was viewing Neptune. I have confirmed that what I was seeing was Neptune, by analyzing the two photos, and comparing it to Stellariumweb on those dates. Here is my full data on the photos.

Pictures were taken with my usual set up.

9/20/2022

10/02/2022

As you can see, I obviously got it. Here is what’s puzzling me, though. Neptune barely moved in that 12 day period. I just now noticed that.

Why is that? I was under the impression that planets moved a little faster, and that they all moved the same speed, was I wrong?

Can someone help me figure this out?

Jared

[SparWeb]

If you look up the orbital period of Neptune, you will find that it's about 170 years or so. From that you can extrapolate that it travels only 1/170 across the sky (among the fixed stars) every year, which is about 2 degrees. Your 2-week time lapse is 1/25 of that, or about 0.1 degree. So... Yup... that's Neptune for you.

[Jnicholes]

If you look up the orbital period of Neptune, you will find that it's about 170 years or so. From that you can extrapolate that it travels only 1/170 across the sky (among the fixed stars) every year, which is about 2 degrees. Your 2-week time lapse is 1/25 of that, or about 0.1 degree. So... Yup... that's Neptune for you.

Well, I feel sheepish. I can’t believe I didn’t think of that!

Oh well, it happens.

Thank you for getting back and teaching me, I appreciate it.

[SparWeb]

...which actually suggests a leaning exercise which FEW amateur astronomers ever attempt, I dare say. I certainly haven't tried, but now that the idea occurs to me... In much the same way that discoverers of new asteroids, comets and other such bodies take many measurements and plot approximate orbits, if interested you could do the same. Take 2 or 3 measurements of the position of a planet over several weeks or months. Make accurate measurements of the position each time, using known stars and catalogged background objects to check your measurements. Then do the math to estimate the orbit, and - ya know - confirm for yourself that Neptune is actually a member of our solar system!

Okay okay, we all did enough problems whose answers were already known in high school. But if orbital mechanics ever interests you, and you aren't afraid of the math, it's a neat skill-building exercise.

[Jnicholes]

...which actually suggests a leaning exercise which FEW amateur astronomers ever attempt, I dare say. I certainly haven't tried, but now that the idea occurs to me... In much the same way that discoverers of new asteroids, comets and other such bodies take many measurements and plot approximate orbits, if interested you could do the same. Take 2 or 3 measurements of the position of a planet over several weeks or months. Make accurate measurements of the position each time, using known stars and catalogged background objects to check your measurements. Then do the math to estimate the orbit, and - ya know - confirm for yourself that Neptune is actually a member of our solar system!

Okay okay, we all did enough problems whose answers were already known in high school. But if orbital mechanics ever interests you, and you aren't afraid of the math, it's a neat skill-building exercise.

Trust me, I’m not afraid of math. It was one of my best subjects!

I think I will try that sometime when the skies are clear for several days. Maybe I’ll try it out on something simple to start out, like Mars or Jupiter, then move my way up to Uranus and Neptune, then move my way up to other things, like astroids and comets.

I need to start small first, then build my skill level up.

Thanks for the exercise. I will definitely try it out when I have a chance.

[SparWeb]

Let us know how it goes!
It's probably a deep (but interesting) rabbit-hole to fall down.

[KathyNS]

I think I will try that sometime when the skies are clear for several days.

Don't worry about needing several clear nights in a row. You will get more interesting results if you measure at weekly intervals, or even monthly if the planet is far from opposition.