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The Sun follows the solar circle (eccentricity e < 0.1) at a speed of about 255 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8.34 kpc [4] about the center of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR. [5] [6]
The Solar System is traveling at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph) within its trajectory around the Galactic Center, [3] a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately 1/1300 of the speed of light.
In 2005, the first measurement was made of the proper motion of the Triangulum Galaxy M33, the third largest and only ordinary spiral galaxy in the Local Group, located 0.860 ± 0.028 Mpc beyond the Milky Way. [20] The motion of the Andromeda Galaxy was measured in 2012, and an Andromeda–Milky Way collision is predicted in about 4.5 billion ...
Figure 1: Geometry of the Oort constants derivation, with a field star close to the Sun in the midplane of the Galaxy. Consider a star in the midplane of the Galactic disk with Galactic longitude at a distance from the Sun. Assume that both the star and the Sun have circular orbits around the center of the Galaxy at radii of and from the Galactic Center and rotational velocities of and ...
The total mass of the Andromeda Galaxy is estimated to be between 8 × 10 11 M ☉ [56] and 1.1 × 10 12 M ☉. [61] [62] The stellar mass of M31 is 10–15 × 10 10 M ☉, with 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the stellar halo. [63]
The Sun, taking along the whole Solar System, orbits the galaxy's center of mass at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph), [168] taking about 220–250 million Earth years to complete a revolution (a Galactic year), [169] having done so about 20 times since the Sun's formation.
The Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few parts in 100 billion (1 × 10 −11). Historically, observations of Venus transits were crucial in determining the AU; in the first half of the 20th century, observations of asteroids were also important.
The Milky Way is approximately 890 billion to 1.54 trillion times the mass of the Sun in total (8.9 × 10 11 to 1.54 × 10 12 solar masses), [7] [8] [9] although stars and planets make up only a small part of this. Estimates of the mass of the Milky Way vary, depending upon the method and data used.