# Apogalacticon and Perigalacticon

What is the length of the apogalacticon and perigalacticon of the Sun and Milky Way? The general terms seem to be apoapsis and periapsis.

My greatest efforts at Googling have failed miserably. If you can provide references as well, please do!

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According to this website we need another 15 million years to the perigalacticon.

Recall the Sun's motion ... 30 degrees toward the galactic centre from circular motion, and 23 degrees upward out of the galactic plane. We will be at perigalacticon (closest to the nucleus) in 15 million years. At apogalacticon the Sun is seven percent farther out.

During the galactic year of about 250 million years our solar system isn't orbiting on a Kepler ellipse around the galactic center, but instead oscillating up and down the galactic plane.

This vertical oscillation cycles at 3.5 times per galactic year.

We are now about 27,000 lightyears away from the galactic center, close to the perigalacticon. The difference between apogalacticon and perigalacticon is a little more than 4,000 lightyears (15% of 27,000 lightyears). The 15% number is calculated from the 7% eccentricity of the orbit by $1.07/0.93=1.15~~$ according to the definition of eccentricity, and Kepler's 1st law.

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So then the apogalacticon would be 27,000 * 1.07 ≈ 29,000 light years and the perigalacticon would be 27,000 * 0.93 ≈ 25,000 light years. This appears to be consistent with the difference of 4,000 light years. Are these results correct? It seems like it was too easy since the constant ±7% is being applied to our current position as opposed to calculating this from the entire orbit. –  pyrospade Mar 2 '14 at 17:22
We are close to the perihelion, so it's more like between about 27,000 (perigalacticon) and 31,000 lightyears (apogalacticon), but with measurement errors of about 1,000 lightyears. Hence it might also be between 28,000 and 32,000 lightyears. –  Gerald Mar 2 '14 at 19:35

This question was once valid, but as of 1994 we found out that our Solar System is part of the Sagittarius Dwarf Elliptical Galaxy, or Sag-DEG (M54), which is the largest of the 30 or so Dwarf galaxies orbiting the Milky Way. We are in a 500 million year long circum-polar orbit. The two apsides are very far above the equatorial plane of the Milky Way. We just passed through the disc and are now about 50 light years above it, and rising towards our northern apsis (north with respect to the Galactic Coordinate System). Our journey will take us 125 million years to reach the top, and 125 million years to reach the disc of the Milky Way again. Ironically, if the Milky Way experiences a 250 million year rotation with respect to our current position in the Orion Arm (remembering that every location of the MW disc experiences a differential rotation with different orbital speeds), then by the time we reach the disc again 250 million years from now, we will basically pierce through the Orion arm very near to where we are right now! Isn't that amazing?

As for your question, and the answer provided by Gerald, we know that the stars orbiting the MW's core do oscillate up and down. This was predicted in 1978 and is due to the Galactic Current Sheet, which is why all of the stars follow each other in a sine wave pattern as they bob up and down. Because of this, we can predict the apsides for most of these stars, meaning the high and low point above the disc. However, these are not "apogalacticon" or "perigalacticon", which technically represent the furthest and closest and points in a satellites orbit, not as it oscillates through the disc, because Satellites don't do that, but around the Galactic core in an elliptical orbit much like a Sun-Grazing comet gets close to and then very far from the Sun. Think the Large Magellanic Cloud, or the 30 known Dwarf Galaxies, or many of the 158 known Globular Clusters.

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Hi! Thank you for your answer. We tend to encourage the use of references. It would be great if you could include those in your answer. You can typeset links using markdown like this [link](example.com). We hope you find this site as amazing as we do. –  harogaston Aug 3 '14 at 15:43
"Sag DEG is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth" en.wikipedia.org/wiki/Sagittarius_Dwarf_Spheroidal_Galaxy . We are not part of it. –  Envite Aug 4 '14 at 11:28
The sun is not part of the Sagittarius dwarf galaxy. Please see astro.virginia.edu/~mfs4n/sgr –  Renan Aug 4 '14 at 16:26