# 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!

## 2 Answers

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.

• 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. Commented Mar 2, 2014 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. Commented Mar 2, 2014 at 19:35

user68441 is in error. We are not part of the Sagittarius Dwarf Elliptical Galaxy. Our velocity perpendicular to the galactic plane is only 4.95±0.09 km/s with respect to the Local Standard of rest.

Each cycle takes about 70 million years with an amplitude of 100pc (Matese et al. 1995), it will be roughly 30 million years before we cross the plane again.

Given the thickness of the disk is roughly 1000LY, the solar system will stay within the disk, unless perturbed by an interaction with another star.

• In his defense, I came across an article that said that as well, which I believed until I came across an article debunking the idea. The debunk article made a stronger argument. Commented Aug 30, 2017 at 17:50