Institute of Astronomy


Ask an Astronomer - Miscellaneous

J1950 and J2000 Epochs

Published on 23/08/2011 

What are the epochs J1950 and J2000 when looking at objects in the sky? And what do I have to put in my telescope program?

Astronomers use different epochs to give coordinates of objects in the sky due to changes in motion due to primarily the precession of the Earth on its rotation axis. Much like a spinning top, as the Earth rotates, it's rotation axis gradually rotates as well although much slower than the daily rotation we see. Because of this precession, the positions of the stars change over time with a small motion every day. On small timescales this motion isn't noticeable however over decades it is. For this reason, astronomers update their coordinates every 50 years to make it simpler when finding objects.

Although for an object you can find coordinates in either J1950 or J2000 (the two most recent epochs), this won't actually be the correct location in the sky today. However, what your telescope program will do will take the coordinates from those epochs and then calculate what they should be today i.e. J2011 and then move your telescope there. This saves people doing things by hand and so means you can find objects with the standard coordinates quickly and easily.

As such, all you should need to do is find the coordinates of the object in either J2000 or J1950 and can then put those into your program. The program will then do the rest.

Stars on the Moon

Published on 02/05/2011 

Is it possible to see stars from the surface of the moon? I thought it was but in an interview in 1970 ( Neil Armstrong said you can only see the Sun and Earth.

When US astronauts visited the moon as part of the apollo program, people often comment in images that you don't see the stars. This is the same phenomenon Neil Armstrong described in the interview on the BBC in 1970.

The stars are actually visible from the surface of the moon but they are particularly difficult to observe. If on Earth you compare a night where there is no moon with a night with a full moon, it is much easier to see stars on the dark night with no moon. The same phenomena happens on the moon with the Earth reflecting the Sun's light as well as the astronauts only working in sunlight. And as you know from experience, during the day time we don't see any stars (unless you know exactly where to look using a telescope!) even though they're still shining.

So although the stars are there, the light conditions when the astronauts were working meant that they were unable to see them.

Neutron Decay

Published on 14/03/2011 

I understand that free neutrons pervade the universe and that they are relatively unstable. What do they decay into?

A neutron is not a fundamental particle, rather it is made of three particles known as quarks bound together. There are six types (or 'flavours') of quark and a neutron consists of one so-called 'up' quark and two 'down' quarks.

When not bound in a nucleus, a neutron is unstable and one of the down quarks undergoes a beta decay (like in some radioactive nuclei) in which it becomes an up quark - this remaining arrangement of 2 up quarks and one down quark is a proton. Therefore, a neutron decays into a proton and this process also emits an electron and an anti-neutrino (a very light, uncharged fundamental particle). The half-life of a free neutron (if you were to have a collection of them, the time it would take for half of them to decay) is around 10 minutes.

Nuclei of Atoms

Published on 14/03/2011 

How are protons and neutrons combined to create the nucleus of an atom?

Protons and neutrons are not a fundamental particles, rather they made of three particles known as quarks bound together.

In addition to electrical forces from their charges, quarks also feel the strong force, another of the fundamental forces. It is this force that binds together quarks into neutrons and protons and also holds the protons and neutrons together in the nucleus of an atom. As the name suggests, this is a very strong force - it is able to hold together protons in a nucleus despite their like charges repelling each other and remarkably, the further apart you move the particles, the stronger the force gets between them!

The colour of the sky

Published on 28/02/2011 

Why is the sky blue during the day then red at sunset?

Light from the Sun contains all colours of the rainbow. When we look directly at the Sun, we see all of the light from it, so it appears white or a faint yellow. Particles in the Earth's atmosphere scatter shorter wavelength (bluer) light more than the longer wavelength (redder) light due to a process called Rayleigh scattering. During the day, the entire sky looks blue, as we are seeing the light that has been scattered towards us from all directions. However, when the Sun's rays travel through more of the denser part of our atmosphere (at sunrise and sunset) it appears red, as more of the blue light is scattered.