Observing the heavens
Surprisingly little equipment is needed to make quantitative observations of the sun, moon, stars, and planets.
The Greek model of the cosmos was largely based on naked-eye observations of the sky.
- The Maya used, as their observatory, a
hole in the ground to come up with the precise calendar in the world in their era.
Copernicus proposed that the sun--not Earth--was at the center of the solar system, based on observations he made by sighting along sticks at the planets and stars.
Some orienting questions to start with...
- How do constellations move across the sky?
- Why is it that Polaris (the "North star") appears to be stationary?
- What's special about the "Zodiac" constellations?
- How do planets move across the sky?
- When is "solar noon" (the time when the sun is the highest in the sky) in Goshen, Indiana?
- What's the "highest" (highest angle) the sun rises to in the sky at this time of year?
- How does that highest angle change during the year?
SkyView app
We'll use an iPad app that overlays current star/planet maps on top of its camera image. So, you can point your iPad at a star and find out what star it is, what constellation it's a part of and more.
Look for the "SkyView Lite" app in the App Store and download it to your iPad. Open the app, and answer these questions together with someone else. Write down your answers.
- Tilt your screen around and see what kinds of objects SkyView will show you. Make a list of some.
- You should run across a "red line" on the screen. Follow it around for a while. What do you find marked on that red line? What do you think the red line represents?
- The circle in the middle of the screen allows you to get information about an object in the circle.
- Use the "hamburger menu" (three horizontal lines), then click "the wrench" | Display | Coordinate Display, and then make sure "Azimuth and Elevation" are checked. Move your iPad around to get to the specified coordinates, and describe what direction your iPad camera is pointed--That is, what direction is it "looking" in:
- Azimuth:0, Elevation:0
- Azimuth:90, Elevation:0
- Which way do you *think* you'll pointing for Azimuth:180, Elevation 0? Write down your guess and check if you're right.
- Starting at Azimuth: 90, Elevation:0, move your iPad so that the Elevation increases, while you keep the Azimuth at 90. What direction do you appear to be moving relative to the red line? (In the same direction as the red line? Perpendicular to the red line?)
- What is the Elevation (ignore the Azimuth) when your iPad camera is pointed straight down (flat on a table)? Nearly straight up?
- Find the sun (you can "search" for it, even though we are inside). What is its elevation and azimuth right now?
- Polaris (the North Star) is supposed to be helpful in finding North. What azimuth angle do you think it has?
- The traditional way to locate the North Star is to find the bright stars in the big dipper (or the "drinking gourd" or Ursa Major), then follow the imaginary line from the two stars at the far edge of the dipper to a bright star. It's in a dim constellation called the little dipper.
Tom Wildoner in EarthSky.org
In Skyview, you can either point your iPad towards North and tilt it up to find Polaris. -OR- "Search" for "Polaris" or "Ursa Minor" and follow the arrows on the central circle until you find it. What is its Azimuth angle and Elevation angle?
- Based on your observations, which of the angles (or sweeps) in this diagram is the azimuth angle? And which one is the elevation angle?
Which of the 2
angles above corresponds to the "altitude angle" in this diagram?
Later You'll measure the elevation and azimuthal angle of the sun as it moves across the sky in the Solar Noon activity.
Latitude and the North Star
Earth spins about its rotational axis: an imaginary line the runs through the South Pole and the North Pole.
Points on Earth's equator are just as far from the North Pole as they are from the South Pole. If you think of a plane that intersects the equator, Earth's rotational axis cuts through that plane, and is perpendicular to it.
Latitude is the angle between the plane of Earth's equator and a line that connects the center of Earth to a point on Earth's surface.
If there was a star directly above the North Pole, on Earth's axis of rotation, then as Earth rotates, we would see all the other stars rotating around that one.
There is such a star, and it's Polaris, the North Star!
In this interactive graph, the light rays from Polaris
(in blue)
are coming towards Earth from Polaris. They're all parallel to the axis of rotation, because Polaris is much further away than even the sun!
The green line shows the direction of vertical, or straight up for someone standing at different lattitudes.
The dashed black line indicates the horizon. Someone standing at a particular latitude would be unable to see anything below the horizon (the dashed line), because Earth is blocking their view in that direction.
Use the graph (at tiny.cc/polaris) and a little bit of geometry to think about, and fill in a few entries in the table below of angles to Polaris. Then use your observation of the elevation angle of Polaris, as seen in SkyView, from Goshen to estimate Goshen's latitude. The elevation angle is the angle above the horizon. The horizon (dashed line) is at right angles to the (green) vertical.
| Latitude (${}^o$N) | Angle away from vertical (${}^o$) | Elevation angle (${}^o$) |
|---|---|---|
| 90 (NP) | ||
| 80 | ||
| 70 | ||
| 60 | ||
| 50 | ||
| 40 | ||
| 30 | ||
| 20 | ||
| 10 | ||
| 0 (equator) |
Check your guess by searching the web to find Goshen's latitude...
Test your understanding
- Imagine that you have travelled south to Mexico. When you get there, you wait until night time, and use the "drinking gourd" to find Polaris. Is its elevation angle above the horizon: a.) higher than 41 degrees? b.) lower than 41 degrees? c.) It's not above the horizon, so you can't see it.
- Answer the same question as above, but imagine that you have travelled to Australia. Or to Norway.
- Look at a map of the world. Find another city in the world where the North star would appear at the same elevation angle (within $\pm$ 3 degrees) above the horizon as Goshen.
- The elevation angle of the North Star when we checked it in class today was about ___ degrees. What would the elevation angle of the North Star be 12 hours later on the same day?
Q: Are the stars in a constellation actually close to each other in space?
A: Stars are actually spread throughout 3 dimensions. Some of the stars in a constellation are very close and others may be very far from Earth. From the point of view of Earth, they appear close together in the same way as the objects below, which are at different distances from the camera!
So the answer is No, the stars in a constellation are not necessarily very close. They might be very far from each other. But they are all very close to the same direction from Earth! Just as these cities are all roughly West-and-a-little-North from Goshen.
A:
The zodiac constellations are spread, roughly evenly, around the "ecliptic"--the path that the sun traces across the stars. Or would trace in front of the constellations, if we could see them in the daytime!
You won't see your constellation on your birthday, because it's right behind the sun on that date! A few months before your birthday you can see it in the West, setting after the sun and close to where the sun set.
A: The ecliptic is the plane of Earth's orbit. Since Earth orbits around the sun, it's also in the ecliptic. In SkyView the there is a dashed line to indicate the ecliptic. So one way to find it in SkyView is to look for the sun, and then you'll see the dashed line going through it. Press the big button in the middle of the right-hand side of the screen to lock it on.
One of the remarkable things about our Solar system is that the planes of the orbits of all the other planets are very close to the plane of our orbit. On any given night, this means that the planets are all fairly close to the ecliptic. And when they are all simultaneously in view, they appear all very nearly along a line, that line being the ecliptic, as in this photo below, (2010, by Jia Hao) taken from Indonesia (close to the equator).
Peter Hartl on flickr
Q: Where should I look to see my zodiac constellation on my birthday?
NASA/JPL-Caltech Follow the link for much more about constellations.
Q: Say more about the Ecliptic...
