# What's the Earth's "absolute" speed? i.e. in relation to the central Sun, the first-order star, or related to the Universe known center of mass...

May 7, 2016

Velocity is always stated with respect to a reference point. It is a relative characteristic of an object. As such the question, though appears simple is meaningless in the present form.

#### Explanation:

What do we mean when we say that a car is traveling at $90 k m p h$? We imply that the car travels $90 k m$ across the Earth in one hour. Remember that we ignore the fact that the Earth itself is moving.

We assume that the Earth is our reference point. We live on earth and it is the center of our world. However, we discovered hundreds of years ago that Earth is not at the center of our Solar System. It moves in an elliptical orbit around the Sun.

• We can assess How fast does the Earth move around the Sun? We can figure this out with simple geometry and obtain that its speed around the Sun of about $1.07 \times {10}^{5} k m p h$.
• Quite recently we discovered that our the Solar System is actually nearer to the edge of galaxy and orbiting around the galactic core.

By measuring the speed of other galaxies with which these move towards or away from us, we assess our own orbital speed to be about $8.28 \times {10}^{5} k m p h$. But we can’t just add this to Earth’s speed around the Sun, as we’re moving in an ellipse.

Perhaps we can say that Earth’s speed around the galaxy is somewhere between $7.21 \times {10}^{5} k m p h$ and $9.35 \times {10}^{5} k m p h$ depending on a particular day of the year.

• Then we discovered that the Milky Way galaxy is not the centre of the Universe. Instead, it is just one in a cluster of galaxies, known as the Local Group. Relative to the centre of this group, the Milky Way travels at about $1.44 \times {10}^{5} k m p h$.
• Now The Local Group cluster is part of a larger structure made of all the neighbouring clusters called The Local Supercluster. Relative to which, our local Group moves at about $2.16 \times {10}^{6} k m p h$.

Next$-$ with the Local Supercluster’s speed we run into a problem. We haven’t yet seen what larger structure the Local Supercluster might be part of, which could be used as a reference point.

Map of our corner of the Milky Way galaxy. The Sun is located in the Orion Arm - a fairly minor arm compared with the Sagittarius Arm, which is located closer to the galactic center.