Is the Earth’s spin slowing, and what exactly is a blue moon?

We explore the curious questions that science can answer

The Earth may be slowing down, but our days will not be getting longer any time soon (Getty/iStock)

Is the Earth’s spin slowing?

Yes the Earth is slowing down, but only slightly. In fact, the amount it is slowing down by is so unnoticeable that we don’t need to worry about it. But we can tell it’s slowing over millions of years. In Jurassic times (65 million years ago) the day was only 16 to 18 hours long. Since then the spin has slowed so the day is 24 hours long.

Is it quiet in space?

Yes. Sound can’t travel in space so all those science fiction movies where there are sounds of rockets blasting away are completely wrong. If you and I were standing in space (if we could do that without space-suits) and I shouted at you, you wouldn’t be able to hear me. 

How big is our galaxy?

Our galaxy is about 50 kilo parsecs. A parsec is 3.26 light years and a light year is the distance light travels in one year. So that makes the galaxy about 1,500 million billion kilometres across.

Why doesn't a spaceship burn up when taking off, just as it does on re-entry?

When the spacecraft takes off it is not moving fast enough to rub hard against the dense atmosphere. This means it does not heat up too much. By the time the spacecraft is going fast enough for heating to occur, the atmosphere it is travelling through is not dense enough to rub against the spacecraft’s sides, so, again, heating doesn’t really happen.

On re-entry, on the other hand, a spacecraft is moving much faster than it is on take-off, so it rubs against the atmosphere much more. It is also gathering speed as the atmosphere grows denser.

Is the Earth getting closer to or further from the sun?

The Earth is neither moving away from nor moving closer to the sun. For the Earth’s orbit to change, there would need to be some gravitational force making it change – like a large nearby planet. Nothing like this exists so the Earth’s orbit stays the same.

Where does the Solar System end?

It is quite difficult to give a precise answer as to where the Solar System ends, but there is a definition for its end point. The edge of the Solar System is the point where the influence of the sun stops.

As you read this there are space probes heading out of the Solar System, looking for its very edge. The sun is emitting a solar wind, which blasts out of the inner solar system. The edge region is called the heliopause; this is where the solar wind hits interstellar gas and stops, creating a standing shock wave around the Solar System. No one is entirely sure just how far away the heliopause is.

Why do satellites move so rapidly across the sky?

To keep in orbit they have to move fast. The velocity at which satellites have to travel depends only on the height above the Earth and the Earth’s mass. If a satellite travels slower than it needs to, it will spiral into Earth. If it moves faster it will break away and head into space.

The International Space Station, for example, orbits at a height of about 400km and takes only 92 minutes to orbit the Earth once. This is pretty fast; the station generally takes only a few minutes to cross the sky. However, some satellites are sent into a geostationary orbit; they still travel very fast but they follow the Earth’s rotation, so from the ground it appears that they are not moving at all.

What is a blue moon?

There are two definitions of a blue moon. The first is a phenomenon caused by ash and dust which is ejected by volcanic eruptions into the upper atmosphere.

When light reflected from the moon passes through the atmosphere, it interacts with the volcanic materials, resulting in a blue casting as seen from the ground. A second definition of a blue moon is when two full moons occur in the same month; this happens once every 2.72 years. The next blue moon is on 31 August 2023.