Our autumn almanac 01.10.2010 – 07.10.2010

The Speed of Light
The month of October begins, its name meaning the eighth is another leftover of the Roman ten month calendar year. The ancient Greek poet Hesiod says of this month that “when the Sun’s strength stops scorching and sweltering, and mighty Zeus sends autumn rain, and people move more comfortably, then by day Sirius passes overhead briefly by day, and travels more at night”. Looking at the night sky this week you will see the star patterns of early autumn that includes the very prominent constellation of Pegasus.

When you look up at the sky at night, if the sky is clear of clouds, and the street lighting where you live is not too bright, you will see stars shining with a brightness that helps us imagine they are much closer to us than they really are. All of the stars we can see in the night sky are incredibly distant from us. Even our nearest star, the Sun, which lights our daylight hours is a distance from us that is hard to understand.

It takes over 8 minutes for a particle of light, or a photon, to travel the 93 million miles between the Sun’s surface and the Earth’s surface. If you were to set out on a journey of this sort of distance, travelling at the average speed we would travel on a car journey, it would take about 300 years.

When you see the Sun setting in the west, the last rays of sunlight you see, just before the Sun sets below the horizon, began their journey on the Sun’s surface over 8 minutes before, even though these photons were travelling towards us at 186,000 miles a second.

The distance it takes for a particle of light to travel for one year is a unit of measurement used in astronomy called a “light year”. This is a mind-boggling distance of 5,879,000,000,000 miles.

Our Sun is one of a group of 100,000 million stars that form the galaxy we see in the night sky as a bright ribbon of stars called the Milky Way. Our galaxy is a spiral galaxy that is so wide that it would take a particle of light travelling at 186,000 miles per second 100,000 years to travel from one edge of the galaxy to the other.

Our Solar System is on the inner edge of one of the spiral arms of the galaxy. This spiral arm is called the Carina-Centaurus arm. This is our galactic address. Our galaxy is always turning and spinning in space, and where our Sun and planet Earth are on the spiral arm, a journey to complete one turn of this star wheel will take about 225,000,000 years. This period of time is called the ‘cosmic year’.


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