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  1. Janus

    I Questions about the Voyager spacecraft

    They are working on all kinds of new propulsion systems. Ion drives, for example. They are much more efficient in terms of fuel usage vs. final velocity than chemical rockets. The problem is that they are very low thrust, very low acceleration and thus take a great deal of time to get up to...
  2. Janus

    I Questions about the Voyager spacecraft

    No, it's due to fuel limitations. There are no refueling stations in space, so the craft has to carry all the fuel its going to ever need. When accelerating a craft, you not only are accelerating it, but also the fuel it will will be burning later. This results in the amount of fuel needed...
  3. Janus

    I Rotation curve with neutral hydrogen and dark matter

    The solar system as a whole has a much higher density than the galactic disk in terms of baryonic matter. So even though the density of DM is roughly the same, the ratio of baryonic to dark matter in the solar system is extremely high compared to the disk as whole. Thus in the solar system...
  4. Janus

    B The other side of the Moon

    Also, there is libration. While the Moon rotates at a constant angular velocity, it doesn't orbit so. Because its orbit is a bit elliptical, its orbital speed varies over the orbit. Thus, its rotation and its orbital speed don't stay exactly matched. So, as seen from the Earth, the Moon...
  5. Janus

    B Speed of light in outer space

    Even if you were considering the coordinate speed of light in deep space compared to that measured at the surface of the Earth, the difference only comes out to be ~ 20 cm/sec. That works out to about a 1/10 of a sec difference in travel time from Alpha Centauri to Earth. However, we don't...
  6. Janus

    I Will stars on the other side of the galaxy affect gravity here?

    While Sagittarius A has a larger effect than that one star, the combined mass of all the matter closer to the center of the Galaxy than the Sun has an effect on the Sun's orbit many magnitudes greater than Sagittarius A alone does.
  7. Janus

    B If the Earth did not rotate?

    If you mean the polar orbit, one problem is that it will tend to preces There's also 3753 Cruithne, which is sometimes called "Earth's second moon" It follows a bean shaped path relative to the Earth, but not one that circles the Earth.
  8. Janus

    B Universal Total Eclipses

    Besides, to use the Moon's gravitational lensing as a telescope, you would need to be at the focal distance from the Moon, which is at some 5300 AU
  9. Janus

    B Universal Total Eclipses

    The point I was making is that in a situation where the orbital distance to planetary radius ratio is relatively low, you have much more leeway with the orbital distance/moon size configuration and still see a total eclipse from somewhere on the planet. So with a moon at Europa orbital distance...
  10. Janus

    B Universal Total Eclipses

    One thing to consider is that The "close fit" of the Moon and Sun comes from the variance of their angular sizes overlapping to some degree. Because the Moon has an elliptical orbit around the Earth and the Earth an elliptical orbit around the Sun, The Moon's angular size varies between 29.4...
  11. Janus

    B If the Earth did not rotate?

    While all Moons will, at some point or another, pass between the Sun and the planet, this does not automatically mean that they cast a shadow on the planet. If the Moon is small, far from the planet, or both, they simply won't block the Sun as seen from the surface of the planet, and won't cast...
  12. Janus

    B If the Earth did not rotate?

    But you, as the Sun, would have to keep rotating in order to stay facing the orbiting Earth. So, if you are rotating, and the Earth always maintains the same orientation to you, that means the Earth is also rotating.
  13. Janus

    B If the Earth did not rotate?

    The interesting thing about Venus' rotation is that at every inferior conjunction with the Earth the same point of Venus' surface points at the Earth. There is some speculation that this might be due to a tidal resonance between the two, so maybe Venus would have locked to the Sun by now if it...
  14. Janus

    B Does matter accumulate in Earth's gravity well?

    The second image starting me wondering if I could go one better. Here's what I ended up with; the 3D representation of a gravity well as an animation with the Earth entering from the right of the frame and exiting to the left. I looped it 4 times for the YouTube video...
  15. Janus

    B Does matter accumulate in Earth's gravity well?

    No. I know that they usually depict a spherical Earth and Sun sitting in their respective dips representing the Gravity wells , but that is just artistic license and not representative of what the model is showing. The warped grid lines are a 2-dimensional model of the 3-d universe. It is...
  16. Janus

    B Does matter accumulate in Earth's gravity well?

    Maybe it will help if we address the exactly what a "gravity well" is. Below is a representation of gravity wells, both the Sun's and the Earth's (not to scale!) The large "dip" is the Sun's gravity well, and the Smaller dip is the Earth's, which itself is in the Sun's gravity well. Things...
  17. Janus

    B Tidal locking for planets of K type (orange) dwarf stars

    Here's the Wiki article on Tidal locking https://en.wikipedia.org/wiki/Tidal_locking It has a formula for estimating the time scale for achieving tidal lock. The time decreases by the square of the mass of the Star And increases by the orbital radius to the power of 6 The mass- luminosity...
  18. Janus

    I Will the Sun end as a Type 1a supernova?

    For a Type 1A supernova, you need to have at least a binary star system. The white dwarf accretes matter from the other star until it builds up enough mass to trigger the supernova. So for a Sun like star to ever produce a type 1A supernova, it has to have a companion star.
  19. Janus

    B When does the perspective from the cockpit of a spaceship change?

    An real approach and landing would look more like this: Coming in on a tangent to the surface, slowing down, and using gravity to curve you around the planet into a landing pattern. You wouldn't come in aimed at the center of the planet.
  20. Janus

    I Astronomy in a Simple Solar System

    Not really, it just takes a telescope with a high enough resolution, with the ability to take photos. You take a set of three photos of the same part of the sky six months apart, and then "Flip" between the photos. If a star appears to jump back and forth between photos( compared to the...
  21. Janus

    I Tunnel to space

    As air flows in the bottom, air builds up in the tube below the projectile. The weight of this air acts against the air flowing in. The pressure differential between air flowing in and air at the bottom of the tube becomes less and less until they equalize. This will happen when the weight...
  22. Janus

    I Does a particular star's brightness change throughout the year?

    And there are such thing as variable stars. Mira is one example. It has a period of ~332 days, and can actually vary enough in brightness to go from not being visible at all with the unaided eye to being one of the brighter stars in the sky.
  23. Janus

    B Largest Moon With A Concave Orbit

    The largest moon in the Solar system is Ganymede, and it has a "looped" heliocentric path. Charon also has such a path.
  24. Janus

    I Are stable orbits possible between Mars and Jupiter?

    Mars is roughly 1.52 AU from the Sun and Jupiter is 5 AU. The asteroid belt, including periphery groups, extends from 1.78 to 4.2 AU. Graphically, it would look something like this, with the red dot being Mars, the magenta one Jupiter and the blue line the asteroid belt. So the asteroid...
  25. Janus

    I Are stable orbits possible between Mars and Jupiter?

    There are plenty of asteroids in stable orbits between Mars and Jupiter, so such orbits exist. There are some orbital radii that won't work however. Look up "Kirkwood gaps".
  26. Janus

    I Trying to grok the draconic year

    It takes ~6798 days for the Moon's orbital plane to complete a 360 day precession. To get the 346 day draconic year, you need to use the formula: Td = 1/(1/Te+1/Tm)* where Te is the period of the Earth's orbit. (to be exact, the sidereal period of 365.256... days vs the Tropical year* of...
  27. Janus

    I What stars were observed @ 1919 solar eclipse to prove relativity?

    On that date, the Sun was in Taurus. The catalogued stars near it were: HR1375 HR1403 67 Tauri 65 Tauri 69 Tauri 72 Tauri
  28. Janus

    I Is it possible for the Moon & a pair of planets to conjunct as smiley face?

    It depends on where in the world you viewed it from. The Moon moves at about 1/2 degree per hour relative to the stars. Thus the time of day you are viewing it on that particular day will determine its relative position with respect to Jupiter and Venus. Where you are in the world will...
  29. Janus

    I Is it possible for the Moon & a pair of planets to conjunct as smiley face?

    On may 16, 2010, this is what you would have seen from Perth, Australia (Sun below horizon). And on Dec 1, 2008, you could have seen this: Not exactly the image being promoted, But I think we can be pretty sure that this recent one is a hoax.
  30. Janus

    I Is it possible for the Moon & a pair of planets to conjunct as smiley face?

    Interesting. World Wide Telescope shows this for the relative positions of Jupiter and Venus for this date ( arrows are sight-line from earth). Celestia shows this Looking over the Earth towards the Moon, neither Jupiter or Venus are shown close, though Neptune is in the picture. You have to...
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