A Down to Earth Guide to the Cosmos

As we saw in Chapter 6, total eclipses are so much more impressive because the Moon is just large enough to block out the light of the Sun’s bright photosphere and reveal the fainter yet stunningly beautiful outer atmosphere, the corona. The material in the corona is affected and moved by the Sun’s magnetic field so it follows the magnetic field lines, in a way that resembles that experiment from school days with a bar magnet and iron filings. The path of visibility of the eclipse tracks along the surface of the Earth across a fairly narrow corridor so the moment of totality lasts for only a few minutes from any one location.

A word of caution though: observing solar eclipses needs very careful consideration. The light from the Sun is intense and anyone looking at it directly is risking serious damage to their eyes. It is only at the moment of totality during a solar eclipse that it is safe to look at the Sun without any protection. At the moment just before the onset of totality, there is still a tiny glimpse of the bright photosphere of the Sun, which even then is still dangerous and can cause damage. My message is simple: unless the Sun is at the moment of total eclipse it is harmful to look directly at it.

There are safe methods you can use and they certainly do not include using the tiny filters that are supplied with cheap telescopes that fit over eyepieces. These are dangerous and will crack under the intense solar energy, allowing the full force of the Sun’s energy to enter your eye. There is a material called Mylar that looks like thin tin foil and, when fitted over the front end of the telescope tube, cuts out enough of the harmful solar energy to allow safe observation. Alternatively you could spend a lot of cash on specialist solar filters or you could project the image of the Sun through binoculars or telescopes onto a sheet of card. You can find more details of this here.

Eclipses of the Moon only happen when it is at the full moon position and eclipses of the Sun only when the Moon is at new moon, yet we do not always see two eclipses each month. As we have seen, the orbit of the Moon around the Earth is tilted with respect to the orbit of the Earth around the Sun, so on many occasions the three objects do not align perfectly for an eclipse and, in reality, the Moon is either slightly above or below the other two objects.

Both types of eclipse are examples of an effect known as syzygy, which means an alignment of three celestial bodies that are bound together by gravity. It is not just the Sun, Earth and Moon that can align in this way though, as it is reasonably common for the Earth, a planet and the Moon or even the Sun to align. We call these events occultations when an apparently smaller body is blocked by an apparently larger one, or a transit when the smaller object moves in front of the larger. Occultation often refers to events in which the Moon occults distant planets, asteroids or stars, but it is possible for planets to occult stars, although these events are much more rare. The last one of these infrequent events was seen back in 1959, when Venus occulted Regulus in Leo.

Lunar occultations that involve stars are regularly timed by amateur astronomers and they help to fine-tune our knowledge of the Moon’s terrain. The lack of a significant atmosphere on the Moon means the light from distant stars is relatively unaffected and almost instantly flicks out when they pass behind the edge, or ‘limb’, of the Moon. Due to the path the Moon takes around the sky there are four bright stars that it can block: Regulus in Leo, Spica in Virgo, Antares in Scorpius and Aldebaran in Taurus. The most impressive lunar occultation occurs when it passes in front of the Pleiades star cluster in Taurus. When seen through a pair of binoculars, the stars of the cluster disappear one by one before slowly emerging on the other side of the Moon.

Identical in nearly every way are the transits, which are still alignments of three celestial bodies but the larger object is at the back. Transits across the Sun are among the most spectacular, but from Earth only transits of the inner planets, Mercury and Venus, are possible. Other types of transits can be seen when satellites pass in front of their parent planet, such as the Galilean moons which can be seen transiting Jupiter at various times. Transits are not as scientifically valuable as they once were, but transits of Venus across the Sun were once used to estimate the distance to the Sun and timings of the transits of Jupiter’s moons were formerly used to calculate the speed of light.

Aside from giving us the backdrop for transits of Mercury and Venus, the Sun is responsible for another phenomenon that comes and goes and peaks with its activity, the aurora. As we saw in the previous chapter, the auroral displays are the result of solar wind, which seems to originate in the Sun’s outer atmosphere.

At an altitude of around 80km the electrically charged particles in the solar wind begin changing the energy state of the molecules of gas in the atmosphere, causing them to glow. As the electrons drop back down to their usual energy level, they must get rid of the energy they gained, which is achieved through emitting light. The wavelength and therefore colour of the light released are determined by the gas molecules giving off the light; for example, the green-brown colour is light being released from oxygen molecules and the blue-red light comes from nitrogen gas. The two gases are the major components of the atmosphere and it is their different atomic structures that give rise to the variation in colours.

Unfortunately, many different factors affect whether an auroral display will be seen, including the location on Earth. The North and South Poles generally will see nightly displays of the aurora, but the further towards the equator, the less the chance of seeing them. The quantity of particles in the solar wind is also of paramount importance and the coronal mass ejections, as they are called, which are the most massive outbursts, can increase the chances of seeing the aurora at lower latitudes tremendously. The only advice for catching the aurora displays, other than heading to your nearest polar region, is to check the various aurora alerting services and, when chances of a display seem high, keep an eye on the horizon towards whichever pole is in your hemisphere; but it is important to avoid high levels of light pollution in that direction as they can mask the show.

The eerie noctilucent clouds are another atmospheric phenomenon that can be enjoyed in the darkening twilight sky. Their name means glowing or shining at night and neatly describes their appearance as they seem to hang against the darkening night sky yet are so bright they seem to glow. They can be seen for a period of up to about eighty days, centred broadly on the summer solstice in each hemisphere, and generally only at between 50 and 70 degrees of latitude. The clouds were first observed in 1885, just a couple of years after the eruption of Krakatoa, although it is unlikely that the eruption had anything to do with their formation. Most likely, more people were looking at the sky at this time and simply noticed the unusual cloud display for the first time.

The clouds are composed of crystals of water ice which form high up in the mesosphere, the layer of atmosphere directly above the stratosphere, and are the fractured edge of a polar weather phenomenon called polar mesospheric clouds. The more familiar clouds seen from day to day form lower in the atmosphere in a layer called the troposphere and it is here that tiny particulates form the nucleus upon which water drops condense to produce the clouds. The crystals that produce noctilucent clouds are thought to form around dust particulates from micrometeors or volcanic eruptions, but they also condense directly from water vapour. The curious thing is that there should be very little water vapour in the mesosphere, as it has less than 100 millionths the amount of moisture than in the driest air found on Earth, in the Sahara desert, so their origin is still a mystery. One theory suggests the water vapour may be lifted up from the troposphere below or is possibly the result of chemical reactions, caused perhaps by human activity. The atmosphere is incredibly thin at the altitude of the noctilucent clouds, around 80km, so the ice crystals form at the very low temperature of –120 degrees. The mesosphere is at its coldest of course over the poles and, curiously, during the summer.

The clouds’ characteristic glow comes from their high altitude, where they are still being illuminated by sunlight in contrast to the deepening twilight at ground level. Their striking blue-white appearance is the result of absorption of the incoming sunlight by the ozone in the atmosphere. It is hard to accurately predict if and how they will appear, but they are usually to be seen between May and August in the northern hemisphere and November and February in the southern hemisphere. Even though the clouds are part of a polar cloud, they are difficult to see above latitudes of 65 degrees because the sky never darkens enough, and lower than latitudes of 50 degrees the clouds are typically hidden below the horizon.

Far from being unchanging, the night sky is full of new and sometimes unpredictable things to see and, in the case of the phenomena covered in this chapter, nothing more than your own eyes is needed to witness them. Let us not forget, though, that the Universe itself is constantly changing; stars come and go, galaxies collide and evolve, and even planets alter their appearance, so there is always something new to enjoy.

July: Northern Hemisphere Sky

Looking at the sky we can see why our ancestors believed the night sky was perfect and constant, but it is not until you examine it more closely that you realize it is changing. Take the stars in the easily recognized formation to the south-east called the Summer Triangle: they appear to remain the same year after year. The points of the triangle are marked by Altair in Aquila to the south, Deneb in Cygnus further north-east and Vega in Lyra due west of Deneb. To the casual observer Vega seems to be the brightest, followed by Altair and then Deneb as the faintest, yet in reality Deneb is one of the most luminous stars known, but at a distance of 1400 light years from us its brightness is diminished significantly.

The most southerly of the stars in the Summer Triangle is Altair, which sits in the constellation of Aquila and is a pale yellow star just over 16 light years away. Because of its proximity to us it moves fast enough with respect to other stars for its position to change by around twice the diameter of the full moon in only 5000 years. The movement of close stars like Altair is caused by their motion through space, but stars can display an apparent motion due to the Earth’s movement around the Sun. This ‘parallax’ shift was first measured for a star called 61 Cygni in the mid-1800s by Friedrich Bessel, who determined its distance to be 10.4 light years, just 1 light year off our modern-day value of 11.4 light years. It is a subtle 5th magnitude star found 10 degrees (one fist’s width at arm’s length) to the south-east of Deneb in Cygnus, forming a triangle with Gamma Cygni at the centre of the cross.

Just to the north of Altair is a red giant star over 400 light years away, and north of this is the faint and discreet constellation called Sagitta, which represents an arrow shot by Hercules. The four brightest stars in the constellation are between 3rd and 4th magnitude, making them easy to spot from dark skies. Scan the skies to the north-west of Sagitta and there is a beautiful cluster of stars known as the Coathanger, and as its name suggests the brightest stars form the shape of an upside-down coathanger. Over thirty stars make up this loose open cluster and it is a real treat for binocular observers.

Further north still is another faint constellation, Vulpecula, which is made up of three stars fainter than 4th magnitude forming a shallow triangle with its brightest star, Anser, at the northern point of the triangle. Through binoculars, this star looks like it has a fainter companion, called 8 Vulpeculae, but this is just a line-of-sight effect, with nearly 200 light years separating the two. Through binoculars or the wide field of a finder telescope, the star at the eastern end of the three brightest stars, 13 Vulpeculae, forms a semi-circle with its flat side to the south. Almost halfway along this flat side is one of the real treasures of the summer sky, M27, the Dumbbell Nebula. It is a type of nebula that resembles a planet in small astronomical telescopes, and hence is called a planetary nebula. M27 was the first object of this type to be discovered and is visible with binoculars, but it is stunning through even modest amateur telescopes. Despite their name, planetary nebulae have nothing to do with planets but are giant stars that have reached the ends of their lives and have lost their outer layers of gas to interstellar space. In the case of M27, its famous dumbbell shape is surprisingly common: magnetic field lines, affected by the rotation of the stellar core that was left behind, have sculpted the form we see today. Binoculars show it as a fuzzy star, while small telescopes reveal a little of the structure of the nebula, but the extra power of a large telescope is needed to uncover the remaining core.

Marking the head of Cygnus almost perfectly in the centre of the Summer Triangle and just to the north of the Coathanger and M27 is a star called Albireo, which to the naked eye looks like a single yellow star. Telescopes or even a good pair of binoculars reveal the beauty hiding from view, a stunning double star system with a brighter yellow star and a fainter blue neighbour. The contrast between the two has led to some people reporting the companion star as appearing purple in colour. The whole system is about 380 light years away but there is some doubt as to whether the two are actually orbiting around each other or just happen to appear close at the moment. If they are gravitationally bound it must take in the order of 80,000 years for the completion of one orbit.

Moving off to the north-west of Albireo we get back to the bright and unmistakable star Vega, the brightest star in the constellation of Lyra and by far the brightest star in that part of the sky. It shines with a distinctly blue-white light, indicating that it is a hot young star with a surface temperature in excess of 10,000 degrees. To the north-east of Vega by just a few degrees is perhaps one of the best-known binary stars in the sky, Epsilon Lyrae. Through binoculars it can be seen to have two components, which orbit around each other, but with the greater ability of a telescope to resolve finer detail the two stars can be seen themselves to be binaries, making this a complex quadruple star system. Even telescopes with an aperture of 10cm can split all four stars, although poor-quality instruments may struggle. There is a fifth component star but this is far too close to one of the others for it to be visible directly.

Moving off to the south-east of Vega is a collection of four fainter stars, in the shape of a small squashed square, or parallelogram, making up the rest of the constellation of Lyra. In between the southernmost stars, Sulafat to the east and Sheliak to the west, is M57, another example of a planetary nebula like M27, although visually they are quite different. Unlike M27, M57 is a much more uniform ring shape, hence its common name of the Ring Nebula. It can just be seen with big binoculars as a faint fuzzy star and telescopes as small as 7.5cm will start to reveal the ring structure.

Extending the line between the two southern stars of Lyra’s parallelogram to the east leads towards Albireo in Cygnus, and directly between Sulafat and Albireo is one of the fainter objects in the Messier catalogue, M56, an 8th magnitude globular cluster like those studied by Harlow Shapley to determine the shape and size of our galaxy.

Another example of a planetary nebula can be seen to the north-west of Deneb and just to the east of Iota Cygni, which marks the western wing of the swan. Much fainter than M27 or M57, NGC6826 shines at magnitude 8.89, making it a challenge to view with binoculars. Its common name is the Blinking Nebula, which refers to the way it pops in and out of sight if viewers shift their gaze to and from the central star and slightly to one side.