Published on November 13, 2007
ASTR1002 Mystery Object Tute: ASTR1002 Mystery Object Tute Patterns in the Night Sky: Patterns in the Night Sky Imagine, once again, that you live in Mog, where the clouds recently cleared (for the first time in history) giving you a view of the sky for the first time. You now know that Mog is a sphere, spinning on its axis and orbiting its Sun. You know that stars appear to circle the celestial pole roughly once a day (actually travelling 361 degrees around in a 24 hour period). But now, you’ve seen something rather strange in the twilight sky. Every two weeks, during the last month or so, you’ve been out just after sunset, looking towards where the Sun has just set. You live on the equator. Slide3: Six weeks ago Slide4: Four weeks ago Slide5: Two weeks ago Slide6: Tonight Explain what you see!: Explain what you see! Your job, working in groups, is to explain what you see. You should thing about the pictures and what they mean for a bit. Once you’ve got everything you can out of them, you should turn to the next page for a few more clues. Clues: Clues The people of Mog are remarkably unobservant. None have ever noticed these particular constellations before. However, you did meet a fisherman who reported seeing a cross of bright stars six months ago. However, he saw the cross at dawn, just above the Eastern horizon. There was no fifth bright star within the cross. All these stars look just like dots to the naked eye - there are no obvious colour differences between them. Some might argue that the middle star in the cross doesn’t twinkle quite as much as the others, but that’s pretty marginal. Telescope Observations: Telescope Observations Mid-way through the tute, I release the following data. It turns out that another group of scientists have recently been observing this part of the sky with their new invention, a collection of lenses they call a “far seer”, or “Tele - Scopos”. They looked at this cross constellation six, four and two weeks ago. Most of the stars in this constellation look much the same when viewed through the “telescope” (they still look like dots of light, only brighter). The middle star of the cross, however, the one that moves with respect to the others, looks quite different. Shapes of the Mystery Object.: Shapes of the Mystery Object. Here is what it looked like six, four and two weeks ago. Six weeks ago Four weeks ago Two weeks ago Enough Clues: Enough Clues That should be quite enough clues to allow you to work out what this thing is. In fact, you don’t really need the telescope data - there’s enough information in your naked eye views of the sky. One last clue - there is an object that behaves just like this mystery object in our own solar system. The next slide gives the answers - but don’t read them until you’ve tried good and hard to figure this out for yourself (the process of figuring this out is far more important and examinable than the answer). The Answer: The Answer The mystery object is a planet, closer in to the Sun than Mog (like Venus in our solar system). As it is just after sunset, you must be here. As you are looking towards where the sun has just set, you must be looking this way. Six weeks ago, the mystery object was here Four weeks ago: Four weeks ago Two weeks later, both the mystery object and Mog have moved around the Sun. The mystery object, being closer in, moves faster. Four weeks ago, the mystery object was here It is now slightly higher in the sky (relative to the background stars) but closer to Mog (and hence bigger) and seen edge-on to the terminator (so it appears half full) More recently: More recently It is now closer to us (and hence larger), but is moving towards the line between mog and the Sun, making it appear lower on the horizon. Its dark side is now facing us, so it appears mostly dark, with only a crescent of light.