Institute of Astronomy


Ask an Astronomer - Miscellaneous

Flying near the edge of darkness

Published on 20/01/2014 

On a flight to Greenland on the 18th  January 2014, we were approx 30 mins from Kangerlussauq, arriving at 9.40 local time.

When we looked out of the right hand window of the aircraft the sky above was blue - daylight - underneath that there was a layer of darkness where we could see the moon and a star and underneath that the 'land' which was a cloud layer.

Out of the left hand widows the sun was rising and looked normal.

Is that possible due to flying close to the 'night and day line' on the earth?

This sort of effect is possible when flying close to the terminator (the line between day and night).  At the time you were over Greenland and saw this effect I presume you were probably at cruising altitude 10-12km up.  Line of sight and the curvature of the Earth means that from that height you could see the Sun, although on the ground below the Sun would not yet have risen. Similarly in the sky on the 'nightward' side of the aircraft the Sun would have been shining high up in the atmosphere, but not nearer the ground, and being high up yourselves you could see the effect more clearly.  On the ground this is why the sky toward the horizon where the Sun has either recently set or is soon going to rise appears lighter.

Meteorite identification

Published on 24/06/2013 

Hello there I was wondering if it is possible to test rocks to see if they are from space? I ask because, this is true! Whilst I was gardening today a rock fell into my wheel barrow.  It sounds crazy, but it did happen.  The object is black, looks like coal, and is very light, I suppose like pumice.  If It didn't fall from the sky, I would say it was a piece of normal earth rock, but as it came from above I'm very curious.

It can be quite difficult to distinguish meteorites from Earth rocks.  If it is a couple of centimetres in size or larger and it were a meteorite then I would expect it to have left a dent in your wheelbarrow.  A black exterior is typical since the outside of the rock would be heated to very high temperatures during it's descent, the exterior is often also pitted and can look obviously like it has been melted.  It would not necessarily have been hot to the touch however since it's passage through the atmosphere is very rapid so the interior remains at the temperature of space (extremely cold), and the exterior will cool very quickly.  The lightness would tend to suggest that it is not a meteorite, since meteorites are more often rather dense and heavy, but that isn't an absolute rule.  If you want more information the Natural History Museum in London houses the national meteorite collection and have an identification service that may be able to help.  If you do have a meteorite you are very lucky!

The speed of rockets in space

Published on 09/02/2013 

I have a question my son wants an answer to. When a rocket blasts out of the earths atmosphere does the speed of the rocket change after it reaches space. In what way?I

That depends upon whether or not its rocket engines are still firing. The rocket engines accelerate the craft. As it gets higher in the Earth's atmosphere there is less air and so less drag to slow it down. Therefore the same amount of push will change its speed more. Note that the Earth's atmosphere doesn't have a hard boundary, it just gets thinner until it is effectively no longer there.

Aside from air resistance, there is another force that acts on a spacecraft: gravity. This will pull it towards the Earth, but its effect gets weaker the further you are away. If you are heading straight up, then gravity will slow you down, unless your engines are pushing you enough to overcome this. Of course, if you start fast enough, you can keep going up even though you are slowing down. Eventually you would get far enough away that the Earth's gravity is no longer important, but you will still have to worry about the gravity of other bodies, most notably the Sun.

If you were to settle into an orbit about the Earth, then gravity just keeps you going in a circle. It doesn't slow you, just changes the direction you are moving. You can think of it as falling with style.

If we were now to consider being in deep space, far from anything, then a spacecraft would keep going in the same direction at the same speed without its engines firing. This is just Newton's First Law of Motion. There are no forces to slow it down, no friction like here on Earth. This is one of the things that science fiction authors often get wrong: if a ship's engines were to fail, it shouldn't shudder to a halt, but continue to coast along at the same speed. 

How can gravity act through empty space?

Published on 26/01/2013 

How can empty space, which has no mass and is therefore not matter, curve? And how can it have an affect on the path of objects? In other words, how can empty space – which is nothing – actually do something (like curve) and how can nothing affect something?

That is an excellent question, and one that is difficult to explain. In general relativity, we talk about gravity being the effect of the curvature of spacetime. It can be difficult to imagine what this really means. There are a number of examples which are commonly used to illustrate a curved space, for example the surface of a sphere. However, when thinking about the surface of a sphere, you normally have the sphere underneath to give it substance. You don't actually need this: the surface can be thought of as a separate entity that can exist whether or not there is a sphere.

When we talk about the curvature of spacetime, what we are really describing are the properties of the metric. This is the quantity that tells us the distance between points. You can define the distance between points whether or not there is anything in between. Try to imagine two objects in a vacuum, even though there is nothing filling the gap between them, the gap could be 5 metres or 5 light-years, and that could be definitely measured. The metric exists whether or not the space is empty. In general relativity we treat the metric as a field, a physical quantity that varies with position. This isn't matter, but it is a something that does exist in a vacuum, and can be thought of as a representation of the gravitational field. You should think of spacetime (the structure of which is given by the metric), rather than a vacuum, as being curved.

Finally, how does the curvature effect matter? Matter always wants to travel in a straight line: what we mean by straight though, isn't what you might usually think of. In this case, we mean the shortest line that joins two points. For a flat space that is straight as you'd normally imagine, but try it on the surface of a sphere and you will get something that looks curved. We call these shortest paths geodesics. It takes a force to push an object off its geodesics, so when travelling unaffected through a vacuum, an object will continue happily along its geodesic. That this might look curved is just an effect of the metric, but the object would have no way of knowing without interacting with something.

I hope that goes some way towards helping you understand. Unfortunately this is a difficult subject. You can test that action at a distance works just by dropping something: it'll travel towards the Earth, even though there is nothing connecting them.

How gravity affects different types of matter

Published on 26/01/2013 

The basic elements of the Earth are not the same as those in the Universe, so how can the Universe have the same gravity?

It was one of Newton's great ideas that the force that makes apples fall from trees is the same as that which causes the motion of the planets. This was quite revolutionary at the time. We believe that gravity is universal, and behaves the same everywhere. We've advanced in our understanding since Newton's time, but a basic principle is that all mass (or energy, as the two are equivalent) interacts gravitationally in the same way regardless of composition.

You are quite correct that the Universe does not share the same composition as the Earth. The most common element in the Universe is hydrogen, at about 74%. This is quite rare in the Earth (about 0.03%), although it is quite common at the surface, being a constituent of water (you are about 10% hydrogen). The second most common element in the Universe is helium, at about 24%. This is exceedingly rare on Earth, though we have managed to find enough to fill the occasional balloon (we're actually running out quite rapidly). The Earth is mostly iron (32%), oxygen (30%) and silicon (15%). However, what type of matter an object is does not influence gravity, the only thing that is important is the mass. The force on a 1 kg mass is the same whatever it is made of, and careful experimentation has verified that.