Institute of Astronomy


Ask an Astronomer - Solar System

Seeing the Northern Lights in the UK

Published on 10/01/2014 

I just wondered if you knew the last time the northern lights had been seen in the UK.

As you can probably guess how often the aurora are seen depends on how far north one is.  In the far north of Scotland it is fairly common to see minor auroral displays, provided one is in a dark location, typically they might be visible several times a year (depending on clouds).  Further south sightings become less common and require a major geomagnetic storm associated with a large coronal mass ejection from the Sun.  The frequency of solar activity varies on a roughly 11 year cycle, with coronal mass ejections large enough to cause significant geomagnetic storms more likely to occur around the peak of a cycle.  The current cycle probably peaked late last year, but has been quite a low cycle without any particularly spectacular outbursts.  The last geomagnetic storm powerful enough to produce aurorae visible from most of the UK occurred around 5 August 2011.  Significantly more powerful storms occurred around the peak of the last solar cycle, especially 14th July 2000 and 28th October 2003, which produced auroral displays visible from the Mediterranean.

Fireball in the sky

Published on 07/01/2014 

I am currently living on Jeju Island off the coast of S.Korea.  On
Friday November 22nd I was on my way out with some college students
when we saw a bright burning light in the sky with a tail.  At first I
thought it was a plane on fire but realized it was not. I took a few
pictures as it moved across the sky.  I am curious to know what we

I suspect that what you saw was a meteor.  Bright meteors can be very
spectacular, and are fairly rare, so you are lucky to have seen it!  The
peak of the Leonid meteor shower, which can produce quite high numbers
of meteors, was around the 17th-18th, so it is possible to one you saw
was a Leonid.

Earth-like Moons part 2

Published on 21/11/2013 

I am a writer and need more info on the question : Earth-like moons

Since the tides are higher would sailing be possible? Also, the seasons; would winter be longer on the dark side of the gas giant?

In and of themselves the much higher tides that one would expect on an Earth-like moon of a gas giant would not be a big problem for sailing.  There would be a lot more small islands, sandbanks, etc. that appear at low tide and disappear at high tide, so it would be necessary to be more careful around the coast and have accurate charts in order to avoid grounding your ship, but out in the middle of the ocean there wouldn't be any issues at all.

The biggest issue would probably not be any direct impact of the tides on the act of sailing, but rather on the ports and shore facilities that are necessary to support fleets of sailing vessels. As an example take what would be a relatively modest mid-ocean tidal range of 5m, roughly ten times that we experience today. Translating that directly into tidal ranges at the coast is not totally straight forward because geographic features can interact with the tide making the local range higher or lower.  For example in the Bristol channel the present day tidal range is up to 9m. Let's assume that here in Cambridgeshire and East Anglia the tidal range is about 10m.  In Cambridgeshire that would mean that at high tide the waters edge would be in the city centre in both Cambridge and Peterborough.  At low tide on the other hand the Wash would be high and dry and the sea would be well over 100km from Cambridge, even Skegness would several 10s of kilometres inland.  This is partly because of the Fens are low lying and the Wash is shallow, but even if we moved elsewhere the effects would still be significant.  If we went to Bristol for example even if we only increased the tidal range to 20m, a much smaller amplification than the Bristol channel experiences today, the Severn would be tidal all the way to Kidderminster and at high tide Cardiff, Newport and Weston-super-mare would all be under water.  At low tide there would be probably no more than 100 metres or so of water separating England and Wales as far down as Exmoor.

The issue of very large tidal plains could certainly be overcome, ships have always generally entered and left harbour at high tide, but it would make things more difficult.

As to the seasons there would not be any substantial effect (assuming that you maintain the same tilt of Earth's axis of rotation).  There would be regular eclipses whenever Earth passed behind the gas giant, but these would only be a few hours every month, not sufficient to have a dramatic effect on climate.

Comet C/2012 S1 (ISON)

Published on 08/11/2013 

If Comet Ison were to break up passing close to the Sun, Would the pieces of the comet emerging from around the Sun, Could those various size pieces somehow Shotgun the Solar System?

The tidal break-up of a comet is, in astronomical terms, a comparatively 'gentle' event and the various fragments of the comet would continue on very similar orbits.  The comet Shoemaker-Levy 9 that impacted Jupiter in 1994 is a very good example.  When it collided with Jupiter in July 1994 it did so as a string of at least 21 fragments as a result of having been tidally disrupted during a previous close passage of Jupiter 2 years earlier.  After the tidal disruption event all of the fragments retained nearly the same orbit such that they all collided with Jupiter over the course of 6 days in July 1994.  More directly similar to the comet C/2012 S1 (ISON) is the Kreutz family of sungrazing comets, which are all believed to have originated from a single large comet that fragmented during a close passage of the Sun.

As a side note, the correct designation of 'comet ISON' is C/2012 S1, the (ISON) appended after the designation just indicates the organisation that discovered it, in this case the International Scientific Optical Network, based in Russia.  It might seem somewhat pedantic, but the ISON team will discover other comets (it is what they do), so there will be other comets with the suffix (ISON).

Blowing up Jupiter

Published on 06/08/2013 

In a hypothetical scenario, if you could bear that in mind I am interested in the effects that would happen to Earth if Jupiter in a hypothetical situation exploded. What kind of magnitude would it be referenced under? And how would the astronomical community report that/those findings back to natons around the world? Would the asteroid belt be perturbed in any way? Shooting out asteroids from it and rocky debris in closer orbit with the gas planet? Your help in this matter to keep your hypothetical hat on just to humour and bear with me would be appreciated, thank you...

It depends in part what you mean by 'exploded'.  The solar system planets are quite widely separated, so at least initially, the effect would be rather small.  The orbits of the other planets would remain close enough to their present orbits that only astronomers would be able to tell the difference, and this will be true whatever type of 'explosion' you want.  As for any immediate, direct effects of the 'explosion', remember that space is a vacuum and so shock waves (sound, earthquakes, etc.) can't propagate.

If we go with a minimal 'explosion' and just magically cause Jupiter to vanish then the only other thing we would have to worry about is what happens to its moons and the asteroids.  The large moons that orbit close to Jupiter would probably all simply be ejected from the solar system and not pose any threat to anyone.  Some of the smaller ones further out might go onto elliptical orbits around the Sun, but there are not that many of them so the chance of a collision would be quite small.  The asteroid belt would likely remain much as it is, interactions with Jupiter actually generally serve to destabilise asteroids rather than the other way around.  The only possible cause for concern would be the Trojan asteroids, which share Jupiter's orbit and lead or follow it by 60 degrees.  These occupy stable regions created by the gravity of Jupiter, so if Jupiter were to disappear some of them might become unstable and come into the inner solar system.  This would likely be a long process rather than a fast one so we would probably have a reasonable length of time to deal with any new asteroid that appeared on a collision course with Earth and not be much worse off than our current situation.  In short, other than the disappearance of Jupiter from the sky, most people probably wouldn't notice the difference.

Now, on the other hand, if you want a 'Death Star' style explosion rather than just a planetary vanishing act things would be a little different.  Jupiter is extremely massive, and all of that mass suddenly flying around the solar system is going to have some rather interesting effects.  Now most of the mass of Jupiter is gas (primarily hydrogen and helium), which the Sun would eventually blow away through X-ray radiation and the solar wind (though it might take some time to do so).  What effect the gas has depends partly on how far the explosion spreads it and so how dense it is, it could potentially exert drag on some asteroids, particularly the Trojans, and cause them to spiral in to the inner solar system.  If Jupiter has a rocky core (we aren't actually entirely certain if it does), then that would cause additional problems.  The core of Jupiter might be as more then 10 times the mass of Earth, by comparison the whole asteroid belt is less than 0.1% the mass of Earth.  Turning all of that into rubble and throwing it across the solar system would cause utter havoc, the conversation between a whole leader and an astronomer would probably go something like this:

Astronomer: Jupiter has exploded, the core, which incidentally we know now was 10 Earth masses, has been pulverized and thrown out across the solar system.
World Leader: That doesn't sound good, is any of it going to hit us?
Astronomer: We haven't determined the exact numbers yet, but we expect the impacts to start within a year.
World Leader: Within a year!?  What kind of damage are we expecting?
Astronomer: It will probably start fairly localised, but the surface will reach saturation quite rapidly.
World Leader: Saturation?
Astronomer: Imagine carpet bombing the entire planet with the largest nuclear weapons you have.
World Leader: Oh... Can we do anything about it?
Astronomer: Nope.
World Leader: So we're doomed then?
Astronomer: Pretty much, yes.

If Jupiter doesn't have a core, or if it is pulverized into small enough pieces in the explosion (think dust), then it might not be so bad.  The gas and dust would probably partially block out the Sun and cause and ice-age, though material raining down into the atmosphere might also cause Earth to heat up, so it might get hotter even with the Sun being dimmed.  Either way the environment would be pretty screwed up but some people might survive.