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How big objects look in the night sky is often described by their angular size. But what exactly does this mean and how is it measured?  The angular size of an object tells us how big something looks in the night sky. If you have two objects which are identical but at different distances away from you, the one which is furthest away will have a smaller angular size. Bigger objects will have a larger angular size than smaller objects at the same distance away. Luckily most celestial objects are roughly spherical so they look like a circle in the night sky. This means 1 angular size is enough to describe the size of the object. Otherwise we would need one for the object’s width and one for its height! You are probably familiar with angle being measured in degrees, with 360 degrees to make a full a circle. In astronomy they work in the same way, with the night sky being 180 degrees - that’s with completely clear horizons. If we divide each degree up into 60 equal slices, each slice represents an angle that is 1 arcminute across. If we then divide an arcminute up into 60 yet smaller evenly spaced slices, those slices each represent an angle that is 1 arcsecond across.  We can measure angles by specifying the number of degrees, arcminutes, and arcseconds that they span. An arcsecond is an extremely tiny angle, it’s 1/3,600th of a degree! Here is a handy trick that you can use to estimate the angular size of something. All you need to know is that if you hold your hand at arm's length, the distance across the end of your pinky finger spans an angle of about 1 degree.  Have a go by looking at the moon. You should gets an angular size of about 30 arc minutes.
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A question often asked is “what is dark matter?” The answer touches all our understanding of physics - from the very large, at the scale of galaxy clusters; down to the very small, at the level of fundamental particles. Our best answer: we do not know. We know something of what dark matter does. But we don’t know what dark matter is. To be concise, I’ll call dark matter ‘DM’ from now on. “Normal matter” interacts with gravity, and particularly with electromagnetic radiation. In everyday terms, if we heat up normal matter, it will emit electromagnetic radiation. If it shines in the visible region, we can see it with our own eyes. This is exactly what we see when looking at the night sky. In complete contrast, DM does not appear to interact with electromagnetic radiation at all. We see only its gravitational effects. So, what kind of evidence do we have pointing to the existence of DM? Let’s look at just three of the many pieces of evidence for the existence of DM. Clue 1: Motion of galaxies in galaxy clusters The first person to postulate the existence of DM was Swiss astrophysicist Fritz Zwicky, in 1933. Zwicky spent most of his life at Princeton and his observations showed that the gravitational attraction between all the visible matter in the Coma galaxy cluster could not account for the observed velocities of the individual galaxies.  The galaxies are moving so fast that they would exceed the escape velocity of the system (as shown in the inset equation) and would therefore fly apart. The cluster of galaxies would not exist, whereas we can see it plainly using telescopes. Zwicky concluded that there must be much more mass than could be seen visually. Since this missing mass is invisible, Zwicky called it “dunkle materie”- DM. And it was not just a little missing mass – it was a massive amount, many times the mass of the visible matter. At the time, many scientists were openly sceptical of this idea. Clue 2: Rotation Curves of spiral galaxies In the solar system with its eight planets orbiting the Sun, the innermost planet Mercury orbits faster than the second planet, Venus. In turn, Earth orbits slower, Mars slower still and so on. This is called Kepler’s second law, or Keplerian motion, after Johannes Kepler, and it was confirmed later by Sir Isaac Newton. Theory indicates that any system where the majority of the mass is at the centre – as with the solar system – will have a rotation curve like this:  In 1975, Vera Rubin, an astronomer at the Carnegie Institution of Washington and her colleague Kent Ford announced the surprise discovery that most stars in spiral galaxies orbit at roughly the same speed, rather than showing Keplerian motion - these galaxies showed a so-called flat rotation curve. The implication of this is that galaxy mass is distributed approximately linearly with radius well beyond the location of most of the stars. The results suggest that at least 50% or more of the galaxies mass is contained in a DM halo around the galaxy extending to a radius of 100kpc or more. The diagram below shows the rotation curve of our own Galaxy, the Milky Way:  From this data, at least out to 8.5kpc, where our Sun lies, the rotation curve is flat, rather than Keplerian; evidence for DM. Clue 3: Gravitational lensing Gravitational lensing – the bending of space time due to large masses, which causes light rays to appear to bend was predicted by Einstein’s General Relativity in 1917.  The blue arcs in Figure 4 show the gravitationally lensed image of a galaxy 10 billion light-years away as it appears through the gravitational lens around the galaxy cluster RCS2 032727-132623 about 5 billion light-years away. However, the amount of lensing is too strong to be accounted for by the mass of normal matter in the foreground galaxy. The mass required is much larger: further evidence for DM.
So what is DM? Come back in a couple of weeks for a possible answer...... You can't miss Venus beaming down as daylight fades. In just a few days time, on Friday 27th March, it reaches dichotomy when it shows a perfect half phase. It is now almost at its greatest elongation from the Sun and won't set until near 11pm. Venus is so bright that a great moment to observe it is whilst there is still some daylight in the sky. It looks like a tiny featureless Moon. Here's a quick iPhone photo I took yesterday in fading daylight through the eyepiece of my telescope:  And Venus is still getting brighter. It is swinging towards the Earth but declining in phase to a crescent. These competing effects reach tipping point at the end of April when Venus peaks in brightness at an amazing magnitude -4.5! Make sure you catch Venus over the next few weeks, it's a beautiful sight.
IF ONLY WE KNEW! As we all do know, forecasting is always difficult, especially of the future. In a maritime climate like ours it’s even harder. In Earth’s atmosphere, most of what we refer to as ‘weather’ takes place at the lower level where we all live – the troposphere. Above this lies the stratosphere. The boundary between the troposphere and the stratosphere is called the tropopause. The precise height of this region depends on our latitude and the timer of year (i.e. where the Earth is in its orbit). In the lower stratosphere, there is a very dynamic wind system known as the jet stream which has a strong influence on our weather down in the troposphere. Jet stream winds can regularly reach over 100mph and are a reason why it can take less time to fly west to east over the Atlantic than it takes to fly east-west. In the images below, the red plots show the strongest jet stream winds. You can obtain a jet stream forecast up tom 10 days into the future at https://www.netweather.tv/charts-and-data/jetstream As a rule of thumb, if the jet stream winds are:
 These are not infallible rules, but do give a strong indication to help plan your observations.
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September 2020
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