21 June 2012

Which is colder, Midnight or Sunrise?

          What is coldest time during a day? Common sense may suggest that it is Midnight and experience would suggest that it is just before sunrise. How many of you ever wondered why is it so? Its a common man's question, no? Sun is our supplier of heat and so the coldest time should be midnight, since that is the time we miss our Sun the most. Then why isn't midnight the coldest of all hours?
          Of course, Sun is our supplier of heat. But the thing is, he doesn't supply it to us directly. The heat (or temperature, loosely) we feel normally is that of the lower atmosphere in contact with the earth's surface. This heat is not the same heat coming from Sun. The energy coming from Sun, when it reaches earth's surface contains majorly visible light which does not have heat content. Our lower atmosphere (or the gases present there) cannot interact with this visible light. Our earth absorbs this heat and gets heated up. Then earth begins to give out this energy in form of longer wavelength radiation called Infra Red (IR) radiation (When you are near a fireside, visible radiation is what you call 'the light' and IR radiation is its warmth). The gases in the lower atmosphere including water vapor and carbon dioxide can absorb this IR radiation and that is what we feel as atmospheric temperature. In short, it is not Sun that gives the warmth we feel, but the Earth. In day time, our Earth stays taking in and giving out radiations, keeping a balance. But during night time there is no taking in, but only giving out. It is this given out heat by Earth that keeps us warm during nights. Otherwise we would have all gone frozen to death. Earth keeps on giving out heat energy without any intake of energy until the Sun shows up again. Now you can guess what happens just before sunrise no? Earth has lost maximum of its energy content and that is why dawn is the coldest of all time. Now it is also clear why isn't midnight the coldest of all hours. Because at midnight, the heat loss from earth is only half along its way to dawn.

14 May 2012

Why shouldn't we use mobile phone at petrol pumps?

          All among us have seen a warning board 'Don't use mobile phone' or something like that in petrol pumps (gas station in US). Haven't we? But do we really know why we should not? Isn't it interesting to notice that this warning is based on a pure myth? Yes, it is.
          The warnings of mobile phone hazards in petrol pumps started circulating by the beginning of 2000's, pointing to a petrol pump fire happened in Indonesia in 1999. The driver of the car was said to be using his mobile phone and the pump got fire. But the truth is, there is no direct evidence available anywhere to prove that such an incident ever happened.  Many other similar baseless stories are circulating widely through internet. There are independent authentic study reports published by American Petroleum Institute, Australian Transport Safety Bureau and Australian Mobile Telecommunications Association all of them saying they could never find a cell phone ever causing a fire. They had chosen around 300 petrol pump fires happened worldwide, for their studies. And so this warning stands purely based on a myth! Almost all the reported petrol  station fires have been caused by static electricity (similar to what happens in clouds in thunderstorms).
          Now a few words on the science of the matter. The risk which the petrol pumps are taking is that of a spark that can set off the petrol to fire and so to explosion. The minimum energy a spark should possess to ignite petrol vapor is around 0.2mJ. It is true that a fully charged mobile phone battery contains around 5 million times this energy. But the problem is, they are not designed to make sparks! If the internal circuitry is very faulty, lithium ion batteries can explode while charging. But how many of us use a mobile phone that is being charged in a petrol pump? Of course, the internal electronics of the phone can make a spark but it will be too small. Why should we bother only about mobile phones when the stereo or pocket-torch you use has higher risk of spark inside? Leave them all, what about the heavy Car battery which is capable of giving you a good shock if you don't handle it properly? Anyway it is becoming customary to blame mobile phones for whatever happens wrong, from infertility to memory loss, right?

(Possibly there are two reasons why mobile companies themselves issue such a warning. One is mobile phones don't come with in-built safety measures against truly hazardous highly inflammable situations. Second is the fear of legal liability if ever anything goes wrong somehow. Anyway let us strictly avoid mobile phones while driving)

22 March 2012

Moon doesn't always rise in the evening!

Most of us think Moon does the night-duty in an office where Sun does the day-duty. Majority of our literature pictures Moon like that; someone who rises in the evening after sunset and sets when Sun rises. But the fact remains, only once in a month our Moon does like that. If you are a good observer without much knowledge in astronomy, you may know that the fact I said is right. But still, you don't know why it is like that. This time we will talk about the Moon's drama up there in the sky.
As you all know, the shape of Moon we see is not always the same. It waxes and wanes in size and shape (വൃദ്ധി-ക്ഷയങ്ങള്‍) called the phases of Moon. Most of us think that this is due to Earth's shadow falling on Moon, don't we? But thing is NOT like that. Earth's shadow has nothing to do with this. It is because, as Moon goes around Earth, from here we see it from different angles on different days. I can make it more clear with a picture below.

Here the situations of Moon being at different positions around Earth is shown. Sunlight will illuminate Moon as it does to Earth. As Moon is a sphere, only half of it will be illuminated at a time (Exactly this is how half of Earth experience day and other half experience night at the same time. When at one side of Earth it is noon, the opposite side of it will have a midnight as shown in picture). As Moon goes once around Earth in every 27 (approx) days, its position around Earth with respect to the Sun rays will be different on every day of this 27 days period. Just take a little time to look at the picture. Seeing from Earth, when Moon is in the direction where sunlight comes from, the part of Moon illuminated by Sun is on the back side and so we cannot see Moon at all. In other words, we are seeing the 'night' in Moon. That day is what we call New Moon. Now consider when the position of Moon is opposite to the direction of Sun, the whole of illuminated Moon is visible to us, giving us a full moon day. (From picture you can also make out that a full moon comes right ahead on sky at midnight) On all other days we can see only a part of illuminated surface of Moon which becomes larger when we go from new moon day to a full moon day. In short it is not the shadow of Earth but the shadow of Moon itself, that causes the phases.
Now let us come to the point in title. First of all, we have to remember that the rise and set of any object on sky is due to the rotation of Earth around its on axis. As we have seen, on a new moon day Sun and Moon are on the same direction and so they rise together. (Yes, Sunrise and Moonrise are together!) But on a full moon day, they are on opposite positions and so Moonrise and Sunset happens together. That means Moon can rise at any time. On a particular day it will rise 5o minutes later it rose the previous day due to its motion in its orbit around Earth.