Distances in Space

How Far Away are Stars & Other Celestial Bodies?

• Use Stellarium to observe the sky and discuss what observations you might be able to use to determine which objects are closest to Earth.

• Do size and brightness always lead to accurate conclusions about the distances between Earth and objects out in space?

Distances in Space

• Objects in space are so far apart that it doesn’t make sense to use units of measurement like kilometers.

• Instead, we use astonomical units (AU’s) or light years to describe these huge distances.

Astronomical Unit• used for measuring “local distances”

… those INSIDE our solar system• 1 AU = average distance from the

center of Earth to center of the Sun (149 599 000 km)

Science In Action p. 379

Light Year = ~10 trillion km

• used for measuring distances outside our solar system

• IT DOES NOT MEASURE TIME – it is the distance light can travel in a year

• light travels about 300,000 kilometers per second. • there are 60 seconds in a minute, 60 minutes in

an hour, 24 hours in a day, and about 365.25 days in a year.

• if we multiply all those numbers together, we come up with approximately 10 trillion kilometers as the distance light can travel in one year.

Views of the Past• because it takes time for light from

an object to travel to Earth what we see is images from the past– light from the Moon is 1.3 seconds old– light from the Sun is ~8 minutes old– light from Pluto is 5 hours old– light from a star in the centre of our

galaxy is 25 000 years old– using the Hubble Space Telescope we

can see images that have taken 12 billion years to reach Earth (…the Earth is about 4.6 billion years old!)

Parallax

• How do we know the distance to various stars and planets?– In 1672, Giovanni Cassini used parallax

to determine the distance from Earth to the Sun

• What is parallax?– Hold a finger up about three to five

inches in front of your nose, focus on a distant object, and then alternately open and close each eye.

– What happens?

With parallax… if you look at the same star from two different location on Earth at the

same time, nearby stars will appear to shift in position relative to other, more distant stars.

Closer Star

Distant Star A

Distant Star B

baseline

OR you can look at the same star from the same place on Earth… but at 2 different

times of the year

Science In Action p.450

Triangulation

• How can parallax be used to measure distances in space?

It involves triangulation which

involves the mathematics of triangles, angles and trigonometry

• 1st - we need to use our knowledge of parallax… take a measurement of an object in space from 2 points on Earth (or from the same point at 2 different times)

• that gives us 2 angles for our triangle

• 2nd - we need to measure the base of our triangle

• 3rd – we need to understand trigonometry …sometime after grade 9!

Let’s try it out…

Instead of using trigonometry… we will create a scale diagram to

figure out the distance to an object!