# Tag Info

Accepted

### How does the Earth move in the sky as seen from the Moon?

Still new at stellarium but here are some quick capture gif lasting one month. Sorry about the quality- limited to 256 colors for smaller gifs. Date on lower left corner. By the way the sun is of ...
• 626

### How does the Earth move in the sky as seen from the Moon?

You are correct. The Earth would always appear in approximately the same location in the sky, when viewed from a point on the lunar surface. And it would be seen to spin, the continents coming in and ...
• 1,003

### Is the science in "Don't Look Up" realistic?

Short observational arcs present difficulties in orbit determination. A couple of examples (taken from Wikipedia) : (392741) 2012 SQ31 was observed for one day, and the best-fit orbit was found to ...
• 87.5k

### Is the science in "Don't Look Up" realistic?

Accurate determination of collision within 1 day of discovery is realistic only if old pre-discovery measurement data is available. We can approximate the orbit determination accuracy with some ...
• 986

### How does the Earth move in the sky as seen from the Moon?

To add to Mark Bailey's answer; the Earth would indeed hang in the sky and rotate, but it would also wax and wane over the course of a lunar day (27.3 Earth-days). Starting at lunar dawn, the Earth ...
• 646
Accepted

### Why is the L1 point (Lagrange) almost 1 million miles from Earth? Shouldn't it be closer to us?

If you divide 333,000 by 10,000, you get 33.3, meaning the Sun should be yanking on an object placed at L1 with more than thirty times the force as the Earth is.... That's not how the Lagrange points ...
• 27.1k
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### "Periapsis" or "Periastron"?

No. These words are English, not Greek. "Periapsis" means the point on the orbit when the two bodies are at their closest. It doesn't matter if this good Greek or bad Greek, it is correct ...
• 87.5k

### How does NASA figure out orbital period and mass data for planets and other celestial bodies?

I'll take a small exception to @JamesK's answer about what NASA does and doesn't do. The Jet Propulsion Laboratory is part of NASA and one of the many invaluable contributions they've made to ...
• 31.6k

### What are "non-Keplerian" orbits? What are some familiar examples in our solar system, and can some still be closed?

What exactly are "non-Keplerian" orbits? Strictly speaking, no orbits are in perfect accordance with Kepler’s laws. Kepler’s laws aren’t really “laws” in terms of physical laws, but are ...
• 13.6k

### What is the difference between the two terms named "Eccentricity" and "Ellipticity"?

Both ellipticity $f$ (also called flattening) and eccentricity $e$ are measures of how elongated an ellipse is, based on the semi-major axis $a$ and the semi-minor axis $b$ (figure from wikipedia). ...
• 13.6k
Accepted

### Orbital velocity of a planet - why is my calculation off by about 10%?

Well done you. I double checked the calculations and couldn't fault what you had done. So I contacted the lead author of the paper about it and here is the response: "After checking the numbers in ...
• 113k

### What are "non-Keplerian" orbits? What are some familiar examples in our solar system, and can some still be closed?

What exactly are "non-Keplerian" orbits? Orbits that don't follow Kepler's laws. Strictly speaking, all orbits are non-Keplerian. In practice, one can model some orbits as basically ...
• 27.1k

### What is Charon's ascending node in reference to?

I'm assuming this is Pluto's equatorial plane. That is incorrect. Charon's right ascension of ascending node with respect to Pluto's equator is undefined. Seemingly paradoxically, it is well defined ...
• 27.1k
Accepted

### Forms of stellar orbits around the galactic center

I'm not sure what the focus is on Sgr A*? Only the stars that are very close to the Galactic center can be said to be "orbiting Sgr A*", the rest of the stars in the Galaxy orbit in the the ...
• 113k

### Could Planet X have a perihelion much closer than 300 AU and still be responsible for the gravitational interaction with 19 TNOs?

Well yes, because Brown & Batygin (2021) say the perihelion would be $300^{+85}_{-60}$ au, so there is roughly a 50% chance that the perihelion is smaller than 300 au according to their work. The ...
• 113k

### How does the Earth move in the sky as seen from the Moon?

To expand a little more, yes the Earth would hang in the same spot in the sky, moving around in a small circle as the moon rotated around it over the course of each of its 28 day orbits. It would have ...
• 1,509
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### Confused on how you are supposed to calculate eccentricity without apsides

The vis-viva equation doesn't care about eccentricity, so no, you can't get it from just the semimajor axis. However, given the radial distance and the velocity vector, and the gravitational parameter,...
• 2,980

### Why isn't Eris considered a planet despite being the body of dominant mass?

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I ...
• 2,390

### What is the difference between the two terms named "Eccentricity" and "Ellipticity"?

Ellipses have a "long radius" called the "semi-major-axis" which is the length from the centre to the ellipse measured along the long axis. And a "semi-minor-axis" which ...
• 87.5k
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### Earth-Moon Barycenter Perihelion

From a comment by the OP, How do I set it to the center of the Sun? Select Vector table as the ephemeris type. Choose the Earth-Moon barycenter as the target body and @sun as the coordinate center. ...
• 27.1k
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### Orbits using Newtons laws

I like to classify solutions of the problem of the time evolution of the complete initial state of a set of objects at some epoch time, where the objects are subject to Newtonian gravitation into two ...
• 27.1k
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### Kepler's equation and eccentric anomaly

Both formulae are correct. The discrepancy is because the formula from the eccentric anomaly article uses the centre of the ellipse as the origin, but the formula from the Kepler's equation article ...
• 8,862

### How does NASA figure out orbital period and mass data for planets and other celestial bodies?

Yes, NASA uses indeed Kepler's 3rd law to calculate the orbital periods of planets and other objects in the solar system. If you go to NASA's Horizons Website and generate orbital data for a planet ...
• 2,304

### Post-Keplerian orbital parameters; is there a generally accepted set with definitions?

I do not think there is a definitive list of post-Keplerian parameters. I would argue there is some ambiguity in the definition of a post-Keplerian parameter. Kramer et al. cite Damour & ...
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### Inclination in Kepler's laws

I am assuming that the question you want answered is how to calculate the elevation of an orbit above a reference plane given the orbital inclination with this plane. If so, please update your ...
• 3,070
Accepted

### Can a binary star optically "orbit" a planet?

Planets don't orbit stars. Stars don't orbit planets. Whenever there are two bodies bound by gravity, they are both orbiting their common center of mass. For example, both the Earth and the Moon ...
• 17.4k

### Clearing the Neighbourhood: Definition of 'Orbital Region'?

The IAU gives no definition, it leaves the term somewhat vague. Various calculations of the "planetariness" of solar system bodies use slightly different notions of the "neighbourhood" Soter's $\mu$ ...
• 87.5k

### "Periapsis" or "Periastron"?

A disclaimer first - I am not an astronomer, but I am a Greek with some ancient Greek language knowledge. "Periapsis" is definitely ancient Greek and it derives from peri+apsi (περί+αψη). ...
• 61