1.5: Numbers in Astronomy
In astronomy we deal with distances on a scale you may never have thought about before, with numbers larger than any you may have encountered. We adopt two approaches that make dealing with astronomical numbers a little bit easier. First, we use a system for writing large and small numbers called scientific notation (or sometimes powers-of-ten notation ). This system is very appealing because it eliminates the many zeros that can seem overwhelming to the reader. In scientific notation, if you want to write a number such as 500,000,000, you express it as Système International d’Unités ). The metric system is summarized in Appendix D (see Example 1.2).
A common unit astronomers use to describe distances in the universe is a light-year, which is the distance light travels during one year. Because light always travels at the same speed, and because its speed turns out to be the fastest possible speed in the universe, it makes a good standard for keeping track of distances. You might be confused because a “light-year” seems to imply that we are measuring time, but this mix-up of time and distance is common in everyday life as well. For example, when your friend asks where the movie theater is located, you might say “about 20 minutes from downtown.”
So, how many kilometers are there in a light-year? Light travels at the amazing pace of
Scientific Notation
In 2015, the richest human being on our planet had a net worth of $79.2 billion. Some might say this is an astronomical sum of money. Express this amount in scientific notation.
Solution
$79.2 billion can be written $79,200,000,000. Expressed in scientific notation it becomes
Getting Familiar with a Light-Year
How many kilometers are there in a light-year?
Solution
Light travels km in 1 s. So, let’s calculate how far it goes in a year:
- There are 60 s in 1 min, and min in 1 h.
- Multiply these together and you find that there are s/h.
- Thus, light covers
- There are 24 or h in a day, and 365.25 days in 1 y.
- The product of these two numbers is h/y.
- Multiplying this by km/h gives km/light-year.
That’s almost 10,000,000,000,000 km that light covers in a year. To help you imagine how long this distance is, we’ll mention that a string 1 light-year long could fit around the circumference of Earth 236 million times.