# Ask an Astronomer - Miscellaneous

## The speed of rockets in space

Published on 09/02/2013
Question:

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
Question:

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
Question:

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.

## Looking back in time

Published on 22/01/2013
Question:

When a picture is taken of deep space and it is said that it is from when the universe was 500,000,000 years old.  Mainly saying that you're looking into the past.  That doesn't make sense to me for the fact that you're able to capture a picture.  Distance and time can coincide but in this case i dont get how this theory works with space?  I understand at such a distance it takes time for light to reach us, the point I'm trying to make is that how can it be said that what we view from deep space is the past not the present?

The effects of large distances and time in astronomy can be a little confusing.  Take as an example Proxima Centauri, the nearest star to our Solar System.  This is 4.2 light years away, which means that it takes light 4.2 years to get from Proxima Centauri to us.  Now since the only way we can see something that has happened at Proxima Centauri is through light, this means anything we see at Proxima Centauri actually happened 4.2 years ago.  If there were a person on Proxima Centauri and they had an exceptionally powerful torch, which the flashed at Earth, it would take 4.2 years for the torch flash to reach us, so by the time we saw it the person would actually have flashed the torch 4.2 years ago.  Now as I said Proxima Centauri is very nearby, when we look at objects in the distant universe they are much farther away, billions of light years, so when we see them we are seeing light that left them billions of years ago, when the universe was much younger.  As a result we can in a way think of looking at objects that are very far away in the distant universe as looking back in time, because the light has taken so long to reach us that the universe has changed a lot in the time it has taken the light to get here.

## Using astronomy to date historical events

Published on 03/01/2013
Question:

I cannot get any clear answers to what should be a simple question. "What percentage accuracy do the ancient astronomers have in fixing dates for the first millennium BCE?"

I am researching the period 1000BCE to 00BCE.
There is a wealth of information about Sumerian and Babylonian astronomical dating.
The "Kings lists" are constantly mentioned.
I simply want to have a reasoned answer to the following:

Allowing for human error, scribal miscopying etc - did the ancients in 1000BCE to 00 accurately observe and plot the planets so that we can retrospectively today recognise that which they recorded and afford it absolutely certain dates (to the nearest five years or so)
Do we know, from the positions that they give a planet in the sky that they called X, that it was that which we call, say, Mars.

Are there examples which clearly establish this e.g.
Carved in stone –
“In the 4th year of King X reign Planet V appeared in the west (whatever) and then moved behind the full moon to disappear in the northeast" - this today our computers tell us is exactly a description of Venus on 4th December 560BCE
Carved in stone –
In the 56th year of the reign of King W planet H did rise in the east and set at midnight one step into the quarter west(whatever) - this is an exact description of Mars on 4th January 500BCE"
Carved in stone –
King X was replaced by King W in his eighth year.

That is - we now know these astronomical events were indeed 60 years apart, they could be seen exactly like that from Babylon and it is clear that this was indeed the 60 years from King X 's 4th year to King W's 56th year.

There is much discussion of accuracy and more of eclipses. The only solid facts must surely come from verifying planet observation and then tracking those dates and comparing them to those given by the ancients.

Or as usual am I being simple minded?

As you have found astronomical observations can indeed be used to help provide support for chronologies of the ancient world, no method is perfect however and there are problems with this.

All astronomical observations one can make related to the Solar system contain many layers of cycles.  Everything of course varies on the cycle of a year as the Earth orbits the Sun, however due to the motion of other bodies and slow periodic changes in Earth's orbit there are also longer period cycles.  For example the times at which Venus rises and sets has a cycle of about 8 years (as well as other longer ones).  In general the shorter cycles are the most prominent while distguishing where an observation falls in a longer term cycle is more difficult.

The end result is that for your example stone carving there might be a match with 560BCE, 552BCE, 544BCE, 536BCE or 528BCE.  Sometimes knowing that it must be one of those dates might be sufficient to combine with other data and pin down an exact match, and sometimes even being able to pin it down within 5 cycles or whatever the case may be might be superior to what was known before, but gaps and inaccuracies in ancient records make it difficult to pin down exact dates from historical astronomical observations.

The most accurate records tend to be those of eclipses (because they are hard to miss and have a short duration), which helps with placing individual eclipses within the longest eclipse cycles and so providing more precise dates.  There are still problems with gaps simply because the further back you go the fewer records have survived whether or not they were originally taken, and scribal errors accumulate over time and are difficult to account for.

An additional problem is that in early history (and even comparatively recently) the calendars were not constant.  Many ancient calendars, such as that used by the Babylonians, were lunisolar with months based on lunar phases plus leap months as required to keep reasonable synchrony with the solar year.  That is fine provided that one knows how the scheme on which the leap months are added, but therein lies the difficulty in that they tend to be adjusted on a more ad-hoc basis or systematically over the medium term but with unknown larger jumps in the long term (think about the break that occurred in England in 1752 with the switch from the Julian to Gregorian calendars).  In fact even today this problem exists in the form of 'leap seconds' added by the international body which supervises global time standards, as they are irregular and unpredictable.

Sorry I can't really give you a simple answer, but I hope this goes some way to at least explaining why it isn't a simple question!