What Happens When An Aeroplane Flies Eastwards At The Speed Of The Earth?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 April 2006

Patric Ngugi and his son Peter cannot agree on the answer to this interesting question: Patrick says, “Imagine an aircraft flying above the earth, moving eastwards at the speed of the earth’s rotation. …the pilot, and passengers in that plane will not feel like they are moving since the plane would be moving at the same speed with the earth’s rotation. So, if the plane flew for 24 hours, it will not have moved from the spot above the airport…

“Now, imagine again, another craft moving westwards at the same speed, i.e., earth’s rotation. How many times will it have gone around the earth in those 24 hours? If the two planes left at the same time going different direction and at that speed after how long will they meet? That is what baffled us. You think you can enlighten us?”

Now, if an aeroplane flies in the manner described by Patrick, It would not be motionless above the airport. The reason is this: when any object is stationary on the ground it is in reality moving at “speed of Earth’s rotation”. Thus when the plane takes off and moves eastward at any speed, it will, in reality, be moving the “speed of Earth’s rotation” plus the speed indicated on its speedometer.

Thus if it moves in the manner described by Patrick it would be actually flying at double “speed of Earth’s rotation”.

Now, this “speed of Earth’s rotation” can be expressed in two ways: first, in kilometres per hour and secondly in degrees per hour. The first value is obtained by dividing the circumference of the earth by the time taken to make one complete rotation. This is about 1,675km/h. The second speed is determined by dividing the total number of degrees around the globe (360) by the time taken to go round once (24 hours). The answer is 15 degrees per hour.

The first speed is only correct for places at or near the equator. As we go farther away from the centre line, the speed reduces. Thus while in Kenya the Earth is rotating at 1,675km/h, in London the speed is only 1,000km/h. The reason is that the distance covered by Kenya in one rotation is greater than that covered by London. If you look at a globe of the Earth, you will notice that the circumference of latitude zero (equator) is greater than that of latitude 51 North (London).

The second speed – degrees per hour – is the same everywhere on Earth. This is because the total number of degrees around any circle (big or small) is the same. Thus all places on the planet traverse 15 degrees per hour.

Now when Patrick says “speed of rotation”, we can assume that he means the 1675km/h. Furthermore, we can also suppose that he is thinking of an aeroplane flying from Nairobi. Even with these assumptions, his reasoning in the first part of his question is not correct. A plane flying due east at that speed will not be motionless above the airport.

Next week, we shall tackle the remainder of Patrick’s questions.