BBC | 23 Degrees

New Moon on Christmas Eve

Sat, 24 Dec 2011 12:00:00 +0000

Distance travelled ~ 919'776'000 km

Ever wondered why the Moon seems to look different at varying times of the month and sometimes, like today, seems to have totally vanished?

(SORRY SANTA NO FULL MOON TONIGHT...)

(Image courtesy of Dry Icons - http://dryicons.com)

These are questions that perplexed mankind for centuries but the answer is actually not all that complicated.

Image credit: US Naval Observatory/Astronomical Applications Department. What does the Moon look like now?

The first thing to understand is that we see the Moon because it reflects sunlight; turn the Sun off and the Moon would to all intents disappear from view.

While the Earth is spinning and orbiting around the Sun, the Moon is orbiting around the Earth, completing one orbit in 27.3 days. Its actually more accurate to say that the Moon AND Earth orbit a common centre of gravity called the barycentre which lies inside the Earth but not at its centre. Because the Moon orbits the Earth, and the Earth orbits the Sun its easy to see that the actual angle between the three objects varies throughout the lunar orbit and its this variation that leads to the 'appearance' of the phases of the Moon.

At the start of this blog, I stated that we see the Moon because it reflects sunlight. If the Moon lies opposite the Sun in the sky then we see the fully illuminated portion of the Moon and see a full Moon. If on the other hand, the Moon is between us and the Sun then we see the non-illuminated portion and see a new Moon. Then at various points between we see a varying amount of dark and light portions as the phases change from full to new and back again. Today the Moon is at its new phase which means its in line with the Sun and can't easily be seen without sophisticated equipment.

You might expect that during either a full or new Moon, we should experience a lunar or solar eclipse every month (the Moon blocks sunlight reaching Earth during a solar eclipse and the Earth blocks sunlight reaching the Moon during lunar eclipses) but it turns out that the orbit of the Moon is tilted by about 5 degrees to the orbit of the Earth around the Sun. On most occasions at full or new Moon, the Moon is either just above or just below the Sun or shadow cast by the Earth, making eclipses a little more rare.

Interestingly if you measure the time it takes from one full Moon to the next it takes 29.5 days instead of 27.3! This strange effect is seen because the Earth is independently orbiting the Sun and the Moon has to travel a little further to get back to exactly the same angle as the previous full Moon.

The short and long of it - December Solstice

Thu, 22 Dec 2011 04:00:00 +0000

Image credit Stu.bloggs/Flickr

Distance travelled ~ 913'772'800 km

Today sees the shortest day in the northern hemisphere and for southern hemisphere dwellers, the longest day! A peculiar idea to get your head round perhaps but it's all actually pretty straightforward. The cause of the changing length of the 'day' (and by this I mean the hours of sunlight, not a 24 hour period) is the same thing that causes the changing of the seasons and we can look to the orbit of the Earth around the Sun for the answer.

Surprisingly perhaps, its not the Earth's distance from the Sun which brings the colder months to the northern hemisphere in fact, the Earth is closer to the Sun during December than it is in July. We need to look at the axis of the Earth's rotation to understand why we experience seasons and changing quantities of sunlight.

The Earth spins once on its axis every day, to be precise once in 23hours, 56minutes and 4seconds which is why stars seem to rise about 4 minutes earlier every day. The axis that the Earth spins around is tilted with respect to its orbital plane around the Sun by just over 23 degrees. During a northern winter, the northern hemisphere is pointing away from the Sun and during the northern summer it's pointing toward it. The opposite is true for the southern hemisphere. This all means that during the northern hemisphere winter, there are less hours when the Sun is above the horizon, indeed around the north pole, there are 6 months of it while the southern pole is basking in 6 months of sunlight.

The exact amount of sunlight we receive at any place on Earth is determined by its latitude. The further away from the equator, the less light is experienced during winter. Even small differences in latitude make a big difference to number of daylight hours for example, London will today experience just 7 hours 49 minutes of sunlight whereas the north of Scotland will experience 6 hours 35 minutes, over an hour of difference. What this means astronomically is that the Sun reaches its southernmost point in the sky today which we call the Winter Solstice, more precisely at 05:30 GMT (or UT - Universal Time). This corresponds to the Sun being overhead at local noon at the Tropic of Capricorn in the Southern Hemisphere.

Saturn's super-storms quite unearthly...

Fri, 11 Nov 2011 18:00:00 +0000

Distance travelled ~ 809'788'800 km

High winds are a common occurrence on Earth but they don't often reach more than 150km/h. The record is held by the Tropical Cyclone Olivia as it moved across Australia in April 1996 which battered the land with gusts of 408 km/h. This is nothing compared to the rather more serene and beautiful looking planet Saturn. High in the atmosphere of this gas giant the wind speeds have been measured at a staggering 1800km/h.

The concept of what causes wind, which is effectively the flow of gas from one place to another, is pretty simple to understand. Take the Earth for example; warmth from the Sun heats the surface which then in turn heats the atmosphere in contact with it. As the air warms, it becomes less dense than the surrounding air causing it to rise which results in an area of low pressure as 'less air' is present. Other surface air will then rush in to effectively fill the void left from the rising air and we experience that as wind.

The storms we see on Earth are just extreme versions of this with areas of particularly low pressure at the centre. Typically they form over the oceans which are a vast reserve of energy. Water is very good at storing and retaining incoming solar energy and its this along with the moisture that gives storms their awesome power.

On Saturn the extreme storms that drive the winds are very similar in structure to those on Earth with low pressure systems but it's the source of the energy which sets them apart. Instead of vast bodies of water, the heat driving the storms on Saturn comes from deep within the planets core. When it formed around 5 billion years ago heat was generated when the pieces from the proto-planetary disk crashed together and its the slow but steady release of this energy which has driven Saturn's super-storms.

Image credit NASA

The Cassini spacecraft witnessed first hand one of Saturn's ferocious storms whilst it was orbiting the planet in December 2010. It was quite lucky given that Saturn is usually relatively storm free, unlike Jupiter however the lucky break gave planetary scientists a unique insight into the local weather system. The images show the storm covering nearly 4 billion square km and analysis of the lightning strikes showed a ten times more flashes than in other storms studied since 2004.

The Orionids are coming

Fri, 21 Oct 2011 13:00:00 +0000

Distance travelled ~ 755'224'000 km

(Mark Thompson's previous blogs for 23 Degrees include a post on the Perseids meteor shower and Jupiter's Great Red Spot. Here he explores octobers's second Meteor Shower, The Orionids.)

Halley's Comet returns to the neighbourhood of the Sun every 76 years, yet each year, around the 21/22 October it's possible to catch glimpses of this famous comet in the Orionid Meteor Shower.

The European Space Agency's Giotto probe penetrated the coma of Halley's Comet in 1986 and took this picture of its nucleus.

There are hundreds of comets that orbit the Sun and each one is a lump of rock mixed ices. When they get closer to the Sun, the growing heat turns the ice straight into a gas in a process called sublimation. We can see this process as comets approach in the growing coma around the rocky core. As it gets closer to the Sun, the solar wind (streams of charged particles from the Sun) push against the coma producing the comet's trademark tail. It's a common misconception that the tail of a comet flows out behind it but in reality, it simply points away from the Sun regardless of its direction of travel. As the comet's journey along its orbit continues it leaves behind a trail of debris. If the Earth happens to move through or close to a comet's orbit in its 365 day journey around the Sun, then it will sweep up some of this material and we will see nature's own firework show, a meteor shower.

At around 22:00 (BST) on Friday evening (21st) the Earth is expected to pass by the densest part of the debris left by Halley and we should see the peak of the shower. However, meteor showers are difficult to predict and always tend to be best observed just after midnight so it's worth keeping an eye out in the early morning hours of the 22nd too. Unfortunately this year, the Moon is a day passed last quarter so its light may affect the number of meteors visible. Even so, there could be up to 30 meteors visible per hour. The best tip to catch a glimpse of this spectacle is to wrap up warm, get outside around 22:00 tonight just before moonrise and watch the sky. Be patient and keep an eye on the sky for the rest of the night just in case it happens later than expected. Happy hunting.

Spacewatch - great red spot on jupiter leaves hurricane irene in the shade

Wed, 24 Aug 2011 16:30:00 +0000

Distance travelled ~ 606'376'800

As Hurricane Irene gathers strength it reminds me of the hurricane that has been raging on Jupiter for at least the last 350 years. Called rather imaginatively, the Great Red Spot (GRS) it was first observed in the 17th Century by Giovanni Cassini through the recently invented called the telescope. Its not just its longevity that brings it whirling into the record books though, its size is also impressive, measuring three times as big as the Earth, compare that to Irene which is about 400km in diameter and you realise quite how big a storm it is.

As Voyager 1 flew by Jupiter in 1979, it captured this photo of the Great Red Spot. Credit/Nasa

Jupiter, unlike Earth, is a planet made up almost entirely of gas with a fluid core but when we look at it through telescopes, its the tops of the dense atmosphere that we can see. Heating from the Sun and from internal sources, drives the convective activity in the atmosphere to produce the familiar high and low weather systems. Just like the Earth, its these high's and low's which effectively cause air to move and produce wind. Its here though that the similarities end. Simple observation from even modest telescopes will reveal strange belt structures in the atmosphere that seem symmetrical in both northern and southern hemispheres.

NASA/Freddy Willems, Amateur Astronomer, July 26 2011

In one of these belts, called the Southern Equatorial Belt, we can readily see the GRS which is an anticyclonic storm (a storm which rotates anti-clockwise) taking about 6 days to complete one revolution. Its elliptical shape seems to be due to the fast jet streams that neighbour it, blowing easterly on the south and westerly to the north. Infrared observations have shown that the temperature of the GRS is lower than that of the surrounding clouds suggesting a higher altitude, estimated to be towering over neighbouring jet streams by 8km. From such a monstrous storm you might expect astronomical wind speeds but surprisingly modest speeds of 430km per hour are measured, compared to Irene's more sedate maximum speeds of 160km per hour.

The colour of the spot isn't even stable, changing from pale pink to a deep red but what causes this colour remains a mystery. We do know that its affected by environmental factors though as the darker central region always appears slightly warmer than the paler, cooler surroundings. Its perhaps the presence of complex organic compounds such as red phosphorous that gives it its distinctive colour but for now, the GRS remains one of the beautiful yet enigmatic mysteries of the Solar System.

Even with the full moon there's hope left in the Perseid meteor shower

Wed, 10 Aug 2011 10:00:00 +0000

Distance travelled ~ 569'660'800 km

GUEST POST

(Mark Thompson is the BBC One Show's Astronomer and when not reporting for them, he is most likely writing about the sky or looking at it! You will often find him zooming in on, or through it, as not only is Mark an enthusiastic astronomer but he is also a qualified pilot. January 2011 saw Mark as part of the successful BBC Stargazing Live Presenting team. He also regularly writes for Discovery.Com and a host of other astronomy and space websites and will be producing a monthly space watch blog for 23 Degrees.)

Image taken by Michael Menefee 7 August, Larimer County, Colorado, US. The peak of the meteor shower should make visible 20-30 per hour.

It may not seem like it but the Earth is hurtling around the Sun at the breakneck speed of about 107,000 km per hour. We don't often get a chance to see evidence of this motion but around the middle of August every year, something happens which gives it away. Planet Earth passes through a field of debris that has been left by Comet Swift-Tuttle as it orbits around the Sun which gives rise to one of natures most amazing spectacles, the Perseid meteor shower.

There are about 30 different meteor showers each year where the Earth passes through the debris left by other comets and they happen at broadly the same time every year. If you watch the meteors carefully during any particular shower, you may notice that they all seem to come from one point in the sky, we call this the radiant. They don't actually come from one point in space, its a visual effect arising from something we call perspective. You can see a similar effect if you stand on a bridge over a motorway, all the parallel lanes seem to come from one point on the horizon. The location of the radiant determines the name of the shower, for example the radiant for the Perseid shower sits in the constellation of Perseus.

During the shower, you can expect to see a number of fast moving lights zipping, unannounced across the sky, all seeming to come from Perseus. These lights are produced as tiny pieces of rock slam into our atmosphere at speeds in excess of 200,000 km per hour. On hitting our atmosphere the gasses in front of the rock get compressed to such a degree that they give off the characteristic flash of light. Typically, most of the pieces of rock (meteoroids) will burn up high in the atmosphere and when they do we call them meteors, if they are big enough to land then they are called meteorites.

This year then, in the early hours of 13th August, we will be treated to the peak in activity of the Perseids although it is possible to see meteors from this shower from now until the end of the month. Unfortunately the Full Moon will be present overnight on 12/13 August which means its bright light will block the fainter ones from our view. The best thing to do is get outside, away from bright street lights anytime before dawn between now and then and you should see plenty. Its best to look for them in the early hours as you are then on the front face of the Earth as it moves through space. Its a bit like all the flies you find stuck to your car windscreen but hardly any on the back window. Wrap up warm and set up a comfortable chair outside, lay back and watch. If you are lucky and you do see some, don't forget to make a wish.

If you plan on viewing the show, send your pictures in to 23degrees@bbc.co.uk