Back to the Sun

I admit to having teetered towards the social science domain in the last post, and sincerely apologise. We had better get cracking with the good stuff.

Let us start from the Sun, a mind boggling ball of plasma (think of it as hot gas for the moment, although not the kind to be taken lightly), constantly spewing energy in a radial fashion (meaning from its centre outwards, in every direction), primarily in the form of heat and light. This energy travels through space, and on its journey, smacks the Earth in the face. Well, it whichever half of Earth’s face that happens to be pointed towards the Sun at that particular moment.

Disection of the Sun
Disection of the Sun (clickable! from HowStuffWorks)

You have very possibly been told that the Sun is responsible for a vast number of natural phenomena observable on our planet, such as photosynthesis, rainbows, and the weather. Of these examples, both photosynthesis and the weather are the results of energy transfers (rainbows are an optical phenomenon); but if the Sun is so far away (namely 1.50×1011 metres to 3 significant figures, also known as an astronomical unit), how does the energy get here? And then how does it spread itself out to perform its duties?

Let us start with how it gets here. More often than not, energy travels far distances in the form of radiation, as waves. The Sun emits radiation across the whole electromagnetic spectrum; these waves all travel through the vacuum of space at the speed of light, (as mentioned in the entry with all the experiments – how long does it take for the Sun’s radiation to reach Earth? See footnotes for the answer). This also works, for example when you are sunbathing; you do not have to be touching the Sun in order to get burnt, so always wear suitable clothing and SPF.

The electromagnetic spectrum (by California Berkeley)
The electromagnetic spectrum (by California Berkeley)

Once the radiation has made its way onto our doorstep, and impacted on the surface, conduction takes over. Somewhat. Conduction is the transfer of (principally) heat from a hot object to a cooler object when they are touching. These objects can be the air and the ground, the sea and the surrounding land, or the hob plate and saucepan sitting on top of it, as well as the soup in the pan! This is why you keep your fingers away from things like hot irons (speaking from experience here).

Conduction - the hand is holding a metal rod; the arrow shows direction of heat flow (from
Conduction - the hand is holding a metal rod; the arrow shows direction of heat flow (from

An interesting thing happens when the heat reaches large quantities of water, like in the oceans. The heat evaporates the water near the top, turning it into warm water vapour, which rises up through the air, above the cool air below. This is called convection, and can result in anything between clouds and hurricanes (see pictures below).

Dry and moist convection (from ABC Channel 13 News Blog)
Dry and moist convection (from ABC Channel 13 News Blog)

Test for convection

Here is a cool demonstration of convection that you can carry out at home. You will need two bottles with identical openings at the top, a hot, coloured, liquid(e.g tea) and a cold, clear, liquid(e.g. water).

Fill both bottles to the top with each of the liquids. Carefully turn the cold bottle of liquid upside down and place on top of the bottle of hot liquid so that the two liquids are in contact and nothing is spilling out. What happens to the system (= the bottles and the liquids)? Tell me in a comment, and see you next time!

Note: It takes approxiamtely 500 seconds, or about 8 minutes, for the Sun’s energy to reach the Earth.


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