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 nineplanets.org 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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Possibility of a Settlement on Deimos and Water on Mars

Question: What is the possibility of an orbital settlement around Deimos as it would have access to the minerals of both the moon itself and the supposed water supply of Mars?  – Colin

Answer: Well, settlements on other planets or moons are many years in the future at this point.  Before deciding to form such a settlement on Deimos, though, the existence of water on Mars needs to be settled.  Note that at this point there is only evidence of surface features that look like the result of flowing liquid.  As to whether it is water or not is still an open question.

Jeff Mangum

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What is the Cause of the Great Red Spot on Jupiter?

Question:  What is the giant red spot on jupiter caused by?  – Abe

Answer: The “Great Red Spot” on Jupiter is a persistent anti-cyclonic storm in Jupiter’s atmosphere, similar to hurricanes on Earth.  It is enormous; about 3 Earth diameters in width, and it has existed for at least 400 years.

Jeff Mangum

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Do the Planets Have a Physical Effect on the Sun?

Question:  We know that our sun has a profound impact on all planets in our solar system. But, does planets have any effect on our sun?  – Vinod

Answer:  Just as the Sun exerts a gravitational pull on the planets, the planets exert a gravitational pull on the Sun.  Since the Sun is far more massive than the planets, the pull felt by the Sun is rather small.  This gravitational pull by the planets, and mainly Jupiter as it is the most massive, actually causes the Sun to orbit around a point just outside its radius.  This would be seen as a “wobble” in the position of the Sun as viewed by a person looking at our Sun from far outside of our solar system.

Jeff Mangum

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Why Do the Planets Orbit in a Plane Parallel to the Spin Axis of the Sun?

Question:  Are orbits in our solar system really moving sideways around the sun instead of up and down around the sun? It’s a perspective/gravity/free-falling thing that’s hard for me to understand sideways.  – Jay

Answer:  First, let me answer a related question.  Why do the planets all orbit the Sun in (nearly) the same plane?   This “co-planar” orbital motion is due to the fact that during the formation of the Solar System from a cloud of collapsing gas and dust the Sun and planets settled into a disk structure.  This disk structure is the result of the conservation of angular momentum which results when a spinning cloud of gas and dust collapses, and represents a balance point between gravitational collapse and the outward force due the spin of the disk (called centrifugal force).  Now, this disk could have been in any orientation, but the most likely configuration would have the residual spin of the disk, including the planets, aligned with the residual orbital spin of the Sun.  This is why the spin axis of the Sun is parallel to the spin axis of the rest of the solar system.

Jeff Mangum

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