Institute of Astronomy


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Ever had a question about astronomy you've want answered? Have a look through the previous questions which we've been asked and if you can't find find your answer, ask us!

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Moon causing earthquakes

Published on 04/04/2011 

I happened to notice that the recent close pass of the moon (supermoon) coincided with the earth quake in Japan. Has any study been undertaken of the effects of the gravitational field of the moon on the tectonic plates on earth? Is there a correlation between earth quake activity and the proximity of the moon to earth?

The particular 'super Moon' phenomenon is a coincidence of the Moon being at perigee (the closest point on its orbit) and a new or full moon.  The Moon completes an orbit and passes through perigee once a month so this close approach of the Moon is not an unusual phenomenon.  The eccentricity of the Moon's orbit (how elliptical it is) is quite small so the difference in the tidal influence of the Moon between apogee (the furthest point in its orbit) and perigee is also quite small.  The influence of the Sun is larger and is responsible for the 'spring' (slightly higher) and 'neap' (slightly lower) tides that occur on a two week cycle with spring tides occuring around new moon and full moon and neap tides occuring around first quarter and last quarter.  By far the largest variation in the tidal effect of the Moon however, and what sets the primary rythm of the tides, is the daily rotation of the Earth.

There have been a number of studies over the years of the effect of the tidal influence of the Moon on earthquakes.  Since the largest variation in the tidal influence of the Moon is the roughly twice daily pattern of high and low tides any effect of the Moon on earthquakes should also show this pattern.  There is some evidence that it is statistically more likely for earthquakes to occur at a time near high tide, when the Moon is overhead or on the opposite side of the Earth, but the link is not very strong.  There is no evidence linking the rather lower level modulations of the tides from the Sun, the eccentricity of the Moon's orbit and the eccentricity of the Earth's orbit to the occurence of earthquakes.  The Earth is a seismically active planet and there are thousands of small earthquakes occuring all the time, which would tend to blot out any signal from the gravitational effects of the Moon.  The Moon on the other hand is a rather seismically inactive body and there is definite evidence from seismometers left on the Moon by the Apollo astronauts that the Moon experiences weak 'moonquakes' as a result of the Earth's tidal influence.  The gravitational influence of the Earth on the Moon is around 100 times larger than that of the Moon on the Earth as well which makes it easier to observe.

To give you a sense of scale the variation in gravity at the Earth's surface due to the main tidal rythm compared to the Earth's gravity is about 1 part in 10 million, in comparison the variation in gravity on the surface of the Moon due to the tidal influence of the Earth is about 1 part in 100,000.

The half-moon terminator

Published on 04/04/2011 

At half moon when the sun is in the sky, why does the terminator of the moon appear not to be perpendicular to a line between the moon and the sun (which it must be)
A perpendicular to the terminator always passes considerably above the sun.

The terminator on the Moon is the line that divides day from night on the moon - the daylight side is that half of the Moon that is closer to, and illluminated by the Sun. The effect you mention is most apparent when the Sun and the Moon are widely separated in the Sky, eg when there is a half-moon. I think the solution to your question lies in the way that we perceive straight lines in a 3-dimensional situation.  For example, if you were to stand on a infinitely long rail track, the parallel rails converge in the distance to either side of you, and thus must appear to curve around you.

We know the Sun, Moon and Earth lie in the flat disc of our solar system, which we see edge-on, so it should appear as a straight line - but this is projected on the sky as an arc. The Sun rises in the east, sets in the west, and its 'straight path' is seen as an arced path when projected onto the sky. The Moon follows the same route across the sky, and the further away it is along that arc from the sun, the more you’ll be off if you’re imagining the path of the light as a straight line across the sky.

Cosmic Expansion Beating the Speed of Light?

Published on 02/04/2011 

When the Universe starts to expand from a single point (singularity) and expands to a size of lightyears in just 10 minutes, then it seems to me that there must have been speeds exceeding the speed of light. Surely that cannot happen?!

Einstein's theory of Special Relativity states that information cannot travel from one place to another faster than the speed of light. When the Universe expands, it is the entire of space and time that is expanding. If you imagine two points in the Universe, as the Universe expands, it is the space between them that expands and everything moves away from everything else, so you will not be able to transmit anything (whether radiation or matter) between them faster than the speed of light (in fact the Universe may be expanding so fast that light travelling from one place to the second may not ever be able to catch up with it, so one place will not be visible to the other!)

Black Hole Formation

Published on 02/04/2011 

Are all black holes formed after the death of a massive star? If not, how are these non stellar black holes formed?

Black holes fall broadly into two categories. Firstly there are galactic, stellar mass black holes, which as the name suggests are found throughout galaxies and have masses similar to that of stars (of order 10 times the mass of the Sun). These form at the end of the lives of stars that are too massive to become white dwarfs or neutron stars. This occurs when they are still greater than two or three times the mass of the Sun by the time they have shed material through stellar winds or supernova explosions during their death. The gravitational force causing them to collapse is too strong for the pressure of electrons or neutrons to support the star as in the case of white dwarfs and neutron stars, so it keeps collapsing down to a black hole.

Secondly, there are so-called supermassive black holes, which tend to be around 100 million times as massive as the Sun. These are believed to be found at the centre of most galaxies about which the stars in that galaxy orbit (in fact our own galaxy, the Milky Way, has a black hole 4 million times the mass of the Sun at its centre). While the exact mechanism of their formation is still unknown, it is believed they form when a large gas cloud collapses to the centre of the large gravitational 'well' in the centre of a galaxy as it forms.

In addition there may be 'intermediate mass black holes' which could explain a number of observed phenomena such as 'ultraluminous X-ray sources.' When matter falls into a black hole, it emits radiation, part of which is seen as X-rays. These appear much brighter than stellar mass black holes, implying their mass is around 1000 times that of the Sun, but they are not at the centre of galaxies. Their formation is a mystery. They are too massive to form from a single star, but they could be formed when multiple stars or stellar mass black holes are pulled together by gravity and merge. Alternatively, they could be the central black hole from a smaller galaxy that has merged with the galaxy in which they are found and in the process, were thrown out from the centre.

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.