While the northern hemisphere leans towards the sun, more direct sunlight reaches us. We call this period summer. 21 June is the day when the North Pole points directly towards the sun, and the tilt is at maximum. The days are longest then as most of the northern hemisphere is exposed. Down in the south, the days are shortest as the Earth itself blocks light from reaching the shivering inhabitants.

      As the Earth moves around the sun, the tilt remains the same. The autumnal equinox (22 or 23 Sept) is the day when day and night are of equal length – twelve hours each, just as they are on the vernal equinox in spring on 20 March. ‘Equinox’ comes from the Latin for ‘equal’ and ‘night’.

      When the northern hemisphere leans away from the sun, less light reaches the surface. This is autumn for us, and eventually winter. Longer days come to the southern hemisphere as shorter days come to the north. The summer solstice of 21 June is also the moment when the sun is highest in the sky.

      The Earth is actually closer to the sun in January rather than June. It’s not the distance – it’s the tilt.

      The best way to demonstrate this is by holding one hand up as a fist and the other as a flat palm representing the Earth’s tilt. As your palm moves around the fist, you should see how the tilt creates the seasons and why they are reversed in the southern hemisphere. Be thankful that we have them. One long summer or one long winter would not support life.

      At the midsummer and midwinter solstices, the conditions can become very peculiar indeed. The summer sun will not set for six months at the North and South Poles, but when it does set, it does not rise for another six. Northern countries such as Finland also experience the ‘midnight sun’ effect.

2. WHY IS IT HOTTER AT THE EQUATOR?

      There are two reasons why the Equator is hotter than the rest of the planet. Strangely enough,the fact that it is physically closer to the sun than, say, the North Pole is not relevant. The main reason is that the Earth curves less in the equatorial region. The same amount of sunlight is spread over a smaller area. This can be clearly seen in the diagram below.

      Also, the sun’s rays have to pass through less atmosphere to reach the equatorial band – and so retain more of their heat.

3. WHAT IS A VACUUM?

      A perfect vacuum is a space with absolutely nothing in it – no air, no matter of any kind. Like the temperature of absolute zero (–273.15 °C /0 Kelvin), it exists only in theory. The light bulbs in your home have a ‘partial vacuum’, with most of the air taken out as part of the manufacturing process. Without that partial vacuum, the filament would burn far faster, as air contains oxygen.

      The classic science experiment to show one quality of a vacuum is to put a ticking clock inside a bell jar and expel the air with a pump. Quite quickly, the sound becomes inaudible: without air molecules to carry sound vibrations, there can be no sound. That is why in space, no one can hear you scream!

4. WHAT IS LATITUDE AND LONGITUDE?

      The Earth is a globe. The system of latitude and longitude is a man-made system for identifying a location anywhere on the surface.

      Latitude takes the Equator as a line of zero. If you cut the world in half at that point, you would have a horizontal plate. The centre point of that plate is at ninety degrees to the Poles above and below it.

      Latitude is not measured in miles but in the degrees between ninety and zero in both hemispheres. London, for example is at 51° latitude north.The curve representing the ninety-degree change is split into imaginary lines called ‘parallels’ – because they are all parallel to each other and the Equator.

      With something as large as the Earth, even a single degree can be unwieldy. For both longitude and latitude, each degree is split into sixty ‘minutes of arc’. Each minute of arc is split into a further sixty ‘seconds of arc’. The symbols for these are:

      Degrees: ° Minutes: ’ Seconds: ”

      With something as large as a city, the first two numbers would suffice. London would be 51° 32’ N, for example. The location of a particular house would need that third number, as well as a longitude coordinate.

      There is an element of luck in the fact that a latitude degree turned out to be almost exactly sixty nautical miles – making a minute of latitude conveniently close to one nautical mile, which is 6,000 feet (1852 metres).

      The longitude of London is zero, which brings us neatly into longitude.

      Longitude is a series of 360 imaginary lines stretching from Pole to Pole. London is zero and 180 degrees stretch to the west or east.

      If the world turns a full circle in a day, that is 360 degrees. 360 divided by 24 = 15 degrees turn every hour. We call the fifteen-degree lines ‘meridians’. (‘Meridian’ means ‘noon’, so there are twenty-four noon points around the planet.)

      Now, this is how it worked. On board your ship in the middle of nowhere, you took a noon sighting – that is, took note of the time as the sun passed its highest point in the sky. You could use a sextant and a knowledge of trigonometry to check the angle. If you were at noon and your ship’s clock told you Greenwich was at nine in the morning, you would have travelled three meridian lines east or west – which one depending on your compass and watching the sun rise and set. You would be at longitude +/–45°, in fact.

      Having a clock that could keep the accurate time of Greenwich even while being tossed and turned on a ship was obviously crucial for this calculation. John Harrison, a clock maker from Yorkshire, created a timepiece called H4 in 1759 that was finally reliable enough to be used.

      All that was left was to choose the Prime Meridian, or zero-degree point of longitude. For some time it looked as if Paris might be a possibility, but trade ships in London took their time from the Greenwich clock at Flamstead House, where a time ball would drop to mark 1 p.m. each day. Ship chronometers were set by it and Greenwich time became the standard. In 1884 a Washington conference of twenty-five nations formalised the arrangement. If you go to Greenwich today, you can stand on a brass line that separates the west from the east.

      On the opposite side of the world, the two hemispheres meet at the International Date Line in the Pacific Ocean. It’s called the International Date Line because we’ve all agreed to change the date when we cross it. Otherwise,

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