…by Anura Guruge
Jupiter, given that it is invariably higher in the sky, will most likely be the bright object that first arouses your curiosity and awe.
If it is later in the night you might see a very bright, burning WHITE star, close to the tree line, maybe even glimpsed through the trees in the East. That would be Sirius, the Dog Star — the brightest star in the night sky. It is actually brighter than Jupiter. This serious brightness of Sirius is due to two factors: it is relatively nearby and it is big — twice as big as the Sun. It is 8.6 light years away. So what we are seeing now, in December 2012, is what Sirius was doing 8 years 7 months ago.
Many never think about this. Light thought it travels fast, still has a speed limit. So if an object is far away, it does take time before light from that object gets to us. Even Sun light takes 8.3 minutes to reach us. The light from Jupiter (i.e., sunlight reflected off Jupiter’s gaseous, yellowish atmosphere) takes nearly 35 minutes to reach us. So in reality when we look up and see Jupiter, we are not actually seeing Jupiter in ‘real time’. We are seeing Jupiter as it was 34.9 minutes ago. Think about that. You can have fun pondering this. ‘Rigel‘, the bright white star that makes up the leg of Orion, is ~860 light years away. So what we are seeing is light that left Rigel 860 years ago. For all we know Rigel could have exploded — supernovaed. It would be 860 years before we would see it.
So the next time you look up at the night sky remember that much of what you are seeing is in time lapse. Light that was there sometime before.