THE CHINGCHOK HUNTER
Natural phenomena controls our calendar
DAVID CANAVAN
How we set the time, count our age and dress for each season are dictated by natural phenomena. The rotation of the Earth and the relation between the Earth and the Sun determine our clocks and calendars.
A day is the amount of time it takes for a planet to make a complete rotation on its axis. On Earth, that amount is 23 hours, 56 minutes and 4.1 seconds, although, due to the Earth's orientation when orbiting the Sun, we round it up to 24 hours.
A day is not to be confused with daytime, which is when the Sun is above the horizon and giving light to the Earth. A day is made up of daytime, the length of which depends on season and location, and night-time, the time when a given area on Earth is facing away from the Sun.
The lengths of the days on other planets vary greatly. The closest planet to the Sun, Mercury, takes 176 Earth days to make a complete spin on its axis! This means one "day" on Mercury is actually longer than its "year", as it takes Mercury only 83 Earth days to rotate around the Sun!
This also means that during "daylight", Mercury has a surface temperature of 467C, whereas its "night-time" temperature is -183C, due to its extremely slow rotation. The fact that an Earth day is so short avoids these temperature extremes and promotes life on Earth.
Time zones
Living in Thailand, I have to be careful with what time I ring my family in England, as they are normally six hours behind us. I also have to be careful when calling my family-in-law in Australia as they are three hours ahead of us. This is because Thailand is east of England but west of Australia. Geographical location determines whether a country faces the Sun earlier or later than another country as the Earth rotates on its axis.
The Earth is split longitudinally into 24 segments, with each exactly an hour apart. Although political, geographical and daylight-saving factors have made these imaginary time lines irregular, the result is that all the areas on Earth have a common 24-hour day. Knowing the exact time of the place that is to be contacted by phone allows people to communicate much more effectively.
Exactly 365.25 days
A year is the amount of time it takes for a planet to rotate around the Sun once. It takes the Earth 365.25 days to rotate around the Sun. This means that the Earth is continuously spinning on its own axis in an anticlockwise direction (when looking down from the North Pole) while it makes its massive orbit around the Sun, which is similarly rotating in an anticlockwise direction.
We always count a year as having 365 days but, as stated above, it actually takes the Earth 365 days and a quarter to make a complete rotation around the Sun. We obviously can't have a quarter of a day, so we simply leave it out until we have four quarters. Then, the four quarters are added as an extra day to the month of February every fourth year - resulting in a leap year.
Four seasons
The seasons on Earth are determined by many factors, which depend on the part of the Earth you live in. Every part has some variations in climate and weather, but certain places are more extreme than the others.
We have different seasons because the Earth is on a tilt relative to its orbital plane. The Earth orbits around the Sun and rotates round a North Pole-South Pole axis. If the axis were upright in its orbit, it would be at 0, but it is actually tilted 23.5 away from 0.
The angle of the tilt remains the same regardless of where the Earth is in its orbit. This means that at some time of the year, the southern hemisphere (the part of the Earth below the equator) is tilted more toward the Sun while the northern hemisphere (the portion above the equator) is tilted away. Six months later, when the Earth has travelled halfway on its orbit, the northern hemisphere tilts toward the Sun while the southern hemisphere tilts away.
Land of the Midnight Sun
Depending on where you are on the globe, this tilt can cause huge variations in weather and temperature. At the Poles, the effects can be dramatic. At some points in the Arctic summer, the northern hemisphere is tilted toward the Sun, which means the sun never sets below the horizon, and the North Pole has 24 hours of daylight! In other words, the sun shines at midnight!
At the same time, it is winter in Antarctica (in the southern hemisphere) and the Sun never rises above the horizon. The South Pole region is not completely dark for 24 hours, as it experiences twilight throughout, but to not see the sun at midday must be very strange!
Seasons in temperate zones In temperate regions such as the UK, there are four seasons: summer, autumn, winter and spring. These are characterised by large variations in daylight hours and temperature. In the winters, temperatures could drop below zero and there could be only four or five hours of daylight in each day. In the summers, the temperatures reach 20C and there could be 17 hours of daylight in each day, while there could be transitional periods in spring and autumn.
Seasons in the tropics
The seasons in the tropics do not fluctuate as much in the amount of daylight and extreme temperatures as the temperate zones. Therefore, the seasons are often split into categories according to weather and climate, such as dry, wet, hot and cool seasons.
In the tropics - whether in the northern or southern hemisphere - the Earth's tilt toward the Sun doesn't much affect the number of daylight hours and temperatures, because the climate in equatorial areas is more stable, with variations only in the amounts of precipitation.
During the hot season in Thailand, you can certainly tell that we are tilted toward the Sun. The rainy season in certain areas of the globe is due to weather patterns that occur as a result of the tilt. However, after returning from Scotland last week, I am thankful that it never gets that cold or that wet here!
David Canavan has an MSc in Behavioural Ecology and teaches science, maths and ICT at Garden International School. David is fascinated by science and loves animals, especially the dangerous kind; the more dangerous the better. You may contact David at davidc@gardenbangkok.com .
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