Does the Sun’s Rotation Speed Dictate the Orbital Speed of the Planets?

Question: Are the planets orbiting the sun at the same speed as the sun revolves on its axis? Or are some planets moving slower or faster than the sun spins, of course the position in the solar system dictates how long it takes to complete one full orbit, but say they were all in the same position close to the sun are they moving at the same speed as the sun spins? When the solar system formed surely the sun dictated the speed at which the dust cloud revolved, so unless objects got bumped to move faster or slower they should all be revolving roughly the same yes??  – Sammy

Answer: The Sun takes about 26 days (at its equator) to 34 days (at its poles) to rotate on its axis at its equator, which is much faster than all of the planets orbit about the Sun.  In fact, the orbits of the planets are dictated (mostly) by the Sun’s gravity and their distance from the Sun.  Also, the speed at which the dust cloud from which the Sun and planets formed was determined by the mass and angular momentum of the cloud itself, as it was much more massive than the Sun and planets.

Jeff Mangum


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What Are Those Two Bright Lights on the Western Horizon?

Question: On July 15, 2015, observing from Las Vegas, Nevada, there are 2 objects (Low) in the Western sky. One is bright the other dim. Are they Venus and Jupiter?  – George

Answer: Yes!  Venus, the brighter of the two, is to the south (left when facing west) of the other bright guy, Jupiter.

Jeff Mangum

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What is that Bright Object in the Western Sky After Sunset?

Question: What bright light is in nepa’s sw sky at 30degrees above the horizion at 9:15 edt.  – Tom

Answer: Although I don’t exactly know what location you are observing from, I suspect that you are seeing Jupiter and Venus, which are both currently visible on the western horizon after sunset.

Jeff Mangum

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Proof that the Oort Cloud Exists?

Question: I’m somewhat skeptical about the existence of the Oort Cloud. Some of the estimates I’ve heard indicate that it could stretch halfway to Proxima Centauri. Unless the interstellar medium is itself far less “empty” than predicted, it would seem unlikely that our star would have such a cosmic structure surrounding it that other stars lacked. Do we have any direct observational evidence that would confirm the existence of the Oort Cloud (evidence not explained by other theories)? Is it possible that the interstellar medium is simply less empty than predicted, and Oort-Cloud-like filler is common in interstellar space?  – Tim

Answer: There is what one could call “indirect observational evidence for the Oort Cloud.  It has been known since 1932 (first proposed by Ernst Opik, then updated by Jan Oort in 1950) that one needs a source for long-period comets that is beyond the orbit of Pluto.  This source of long-period comets, which are gravitationally-bound to our Sun, cannot be interstellar.  Also, we have seen Kuiper Belts (debris orbiting at distances from 30 to 50 AU) around other stars, and as the Oort Cloud is likely a continuation of the Kuiper Belt around our Sun, these properties appear to be common remnants of the star formation process.  Note that our abilities to detect small bodies (a few to 10s of kilometers in diameter) in the Kuiper Belt and Oort Cloud regions are improving, using techniques such as background star occultation, making the ultimate characterization of the Oort Cloud inhabitants in the next few decades a good possibility.

Jeff Mangum


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Why Can’t Venus be Fully Lit by the Sun in the Ptolemaic System?

Question: The first known observations of the full planetary phases of Venus were by Galileo at the end of 1610 (though not published until 1613). Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system (which would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the sun, which is impossible if its orbit is, as the Ptolemaic system requires, between the Earth and the sun). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other geoheliocentric models. [From the Wikipedia page on the Phases of the Venus].

Can you please explain this?  In the Ptolemaic system if Venus orbits the earth why would it not be fully lit when it is on the far side of its orbit around the earth just like the moon when it is not between the earth and the Sun.  – Joe

Answer: In the Ptolemaic system the order of the planets, the Sun, and the stars from the Earth was as follows:

  1. Moon
  2. Mercury
  3. Venus
  4. Sun
  5. Mars
  6. Jupiter
  7. Saturn
  8. Fixed Stars

A key fact in this discussion is the fact that in the Ptolemaic system Venus is always between us and the Sun.  Therefore, we would always see some of the night-side of Venus (the side which faces away from the Sun).  This arrangement would never allow Venus to be fully-lit.  Therefore, the fat that Galileo observed the full-phase of Venus required that Venus orbited the Sun between the Earth and the Sun.

Jeff Mangum


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What if the Sun did not turn into a Red Giant and the Moon moved out of its Orbit Around the Earth?

Question: What effects would we see on Earth if the Sun wouldn’t turn into a Red Giant? If the Moon would move away slowly until extracted from its orbit, would that, for example, affect Earth’s axis?  – Walle

Answer: If our Sun did not turn into a Red Giant, it should slowly cool over many millions of years, thus providing less and less energy to the Earth.  Regarding your second question about the affect that changes in the Moon’s orbit would have on the Earth, it would certainly affect the size and frequency of ocean tides.  The Moon also has a stabilizing effect on the tilt of the Earth’s axis, so if it moved farther away from us the variations in the tilt of the Earth’s axis would likely become more extreme (i.e. the Earth’s axial tilt would be less stable with time).

Jeff Mangum

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Do Large Stars Possess Correspondingly Large Planets?

Question: If VY Canis Majoris has a Solar System, would the planets be star-sized and the moons planet sized?  – Monte

Answer: With a mass of about 30 times that of our Sun, VY Canis Majoris is one of the largest stars known.  The size of a star, though, does not necessarily correlate with the size of the planets that might form around that star.  It is equally possible to find small planets orbiting large stars as it is to find large planets orbiting small stars.

Jeff Mangum

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Equation for Orbits?

Question: Why does the following equation seem to predict stable orbits around the sun as well as for moons around planets without any involvement of balancing centripetal and gravitational forces.

The following is the equation that in my investigation seems to work for planetary motion just using geometric data.

C* Vs^2*Rs^2=Vp^2*Rp

C = 8*G^0.5 = 6.548E-5

Vs  = surface velocity of rotating sphere (i.e. sun)

Rs     = radius of rotating sphere (i.e. sun)

Vp       = orbital velocity of body orbiting the sphere (i.e. planet)

Rp       = distance of orbiting body from the center of the rotating sphere.

G = Newton’s gravitational constant

– William


Answer: I am not sure how you derived this equation, but as it is not dimensionally consistent, it does not appear to be correct.  Just checking the units of the left and right side of the equation, where Newton’s gravitational constant has units (using the cgs system) cm^3/(g*s^2):

cm^3/(g*s^2) * cm^2/s^2 * cm^2 = cm^2/s^2 * cm

cm^7/(g*s^4) = cm^3/s^2

As the units for the left and right side of the equation do not equate, your equation is not correct.


Jeff Mangum

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Location of a List of Solar System Objects and Their Naming Conventions?

Question: Simple question.  What is the website address that I may gain access to the recorded bodies and there naming conventions?  — Chuck

Answer: The International Astronomical Union (IAU) maintains a web site which contains all of the information regarding the recommended naming of astronomical objects.  Within this web site you can also find links to other web pages that contain information on known solar system objects (comets, asteroids, near-earth objects, etc.)  As for a list of solar system objects which concentrates on planets, moons, and asteroids (small bodies), I like the site.  This web site contains detailed descriptions of all of the major bodies in the solar system.

Jeff Mangum

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Star and Planet Sky Location Information

Question: Could you generate an image of the planetary alignment on September 26th,  2014?  – Branden

Answer: I think that you are probably looking for night sky planet positions for a given date.  One of the best sources for this information is the Sky and Telescope This Week’s Sky at a Glance.  Each week Sky and Telescope produces a nice summary of bright star and planet alignments that can guide you as to where to look for these objects in the sky.

Jeff Mangum

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