In this lab, you will learn how to:

Computer with Internet connection

Galileo did not invent the telescope, but he was the first to point it to the heavens. Beginning in 1609, this led to four revolutionary discoveries.

1. He discovered craters and mountains on the moon.

2. He discovered spots on the sun.

Both of these were revolutionary in that the Catholic Church had long ago adopted the Aristotelian idea that the moon and sun were "perfect" objects—spherical, perfectly smooth, and in the case of the sun, unblemished.

3. He discovered four moons orbiting Jupiter.

This was revolutionary in that it contradicted the geocentric model of the universe—the idea that Earth is at the center of the universe and everything orbits it. The geocentric model had also been long ago adopted as truth by the Catholic Church.

4. He discovered the phases of Venus.

Although lesser known than the discovery of Jupiter's moons, the discovery of Venus's phases is more significant, in that it not only contradicted the geocentric model of the universe, it confirmed a prediction of the heliocentric model of the universe—the idea that the sun, not Earth, is at the center.

In Aristotle's (and later Ptolemy's) geocentric model of the universe, Venus rides on a circle called an epicycle and the center of Venus's epicycle rides on a circle called a deferent around Earth.
The center of Venus's epicycle is always on a line between Earth and the sun. Consequently, Venus never wanders too far from the sun in the sky, which is what is observed.
The geocentric model of the universe makes specific predictions about Venus's phases: (1) Venus's phase is new or very close to new when it is closest to Earth and consequently appears largest; (2) it transitions to crescent as it recedes from Earth and consequently appears smaller; (3) it transitions to new or very close to new again when it is farthest from Earth and consequently appears smallest; (4) it transitions to crescent again when it is approaching Earth and consequently appears larger; and (5) it transitions to new or very close to new again when it is closest to Earth and consequently appears largest.

These phase and angular diameter transitions correspond to a curve in the following plot. The curve is thick because different versions of the geocentric model make slightly different predictions.
In the heliocentric model of the universe, Venus's orbit is interior to Earth's. Consequently, as in the geocentric model, Venus never wanders too far from the sun in the sky.
However, the heliocentric model of the universe makes very different predictions about Venus's phases: (1) Venus's phase is new or very close to new when it is closest to Earth and consequently appears largest; (2) it transitions to crescent and then quarter and then gibbous as it recedes from Earth and consequently appears smaller; (3) it transitions to full or very close to full when it is farthest from Earth and consequently appears smallest; (4) it transitions to gibbous and then quarter and then crescent again when it is approaching Earth and consequently appears larger; and (5) it transitions to new or very close to new again when it is closest to Earth and consequently appears largest.

These phase and angular diameter transitions correspond to the red curve in the following plot. Clearly, the predictions of the heliocentric model differ from those of the geocentric model.
Here's a video of Venus in the geocentric model of the universe. It plays fast. Play it enough times to see all of the phase and angular diameter transitions.

Here's a video of Venus in the heliocentric model of the universe. Again, play it enough times to see all of the phase and angular diameter transitions.

In Section C of the procedure, you will measure Venus's phase and angular diameter at different times and then use this information to distinguish between the geocentric and heliocentric models of the universe.

Kepler showed that planets orbiting the sun and moons orbiting planets do not travel on circles, but on ellipses, with the central body at one of the two foci.
The long axis of an ellipse is called the major axis. Half of the major axis is called the semi-major axis, which is usually denoted a.

Kepler also showed that the time that it takes for a planet to orbit the sun or for a moon to orbit a planet—the orbital period, which is usually denoted P—is related to the semi-major axis in the following way. where the value of the constant depends on the central body: All planets orbiting the sun have the same constant. All moons orbiting Jupiter share a different constant. The moon orbiting Earth has yet another constant.

Newton showed that the value of the constant depends on the mass of the central body. (This assumes that the mass of the central body is significantly greater than the mass of the orbiting body.)

Now, consider the case of Earth orbiting the sun. Then P = 1 year, M = 1 solar mass, and a = 1 AU (astronomical unit = 1 sun-Earth distance). Hence: Dividing this equation into the previous equation yields: Solving for the mass of the central body yields: Hence, by measuring the semi-major axis of a moon's orbit around a planet in AU and its orbital period in years, you can measure the mass of the planet in solar masses. In Sections A and B of the procedure, you will measure the orbit of a moon around one of the four Jovian planets and then use this information to measure the mass of that planet.

In this tutorial, you will learn how to monitor an object over an extended period of time.

Use Skynet to monitor an observable Jovian planetary system every clear night for two weeks. Select Jupiter, Saturn, Uranus, or Neptune.

Remember to confirm that the planetary system that you selected is indeed observable from CTIO or SSO this time of year.
If it is not, select a different planetary system.

Select as many of PROMPT-1, 3, 4, 5, and 6 and PROMPT-SSO-1, 2, 3, and 4 as you can, but do not select PROMPT-2, 7, or 8.

Select filters and request exposure durations that will allow you to detect as many moons as possible without overly saturating the planet.

System	Filter	Exposure Duration (seconds)†
Jupiter	V	0.03
Saturn	HiThru	0.03
Uranus	HiThru	1
Neptune	HiThru	1

Request two exposures (just in case one disappoints) and below this select "Show Advanced Time Settings". Under the advanced time settings, request "Repeat this observation every 1 day a total of 14 times" and select "If interrupted continue on next free telescope".

In this tutorial, you will learn how to select and align images, which must be done before they can be made into a movie.

Select and align your images. Remember:
In this tutorial, you will learn how to turn selected and aligned images into a movie.

Turn your selected and aligned images into a movie. Remember: Repeat #1 so the moons can be seen orbiting the planet. In the "Make Movie" window, select either the "avi" or "mov" file format and save your movie by clicking "Download Movie". Check your saved movie by playing it with other software on your computer.

Question: Upload your final avi or mov movie. (Submit a file with a maximum size of 8 MB. 5 points.)

Question: Which planet did you observe in Section A of the procedure? (1 point)

Identify the sharpest looking image of your images. Open it in Afterglow, select the "Histogram" window, and select "MinMax". Zoom in until the planet fills your window.

Question: Measure the angular diameter (θ) of the planet to the nearest 0.1 arcseconds. If your planet is Saturn, measure the angular diameter of its rings. (2 points)
arcseconds

Note: This value should be larger than the true value due to atmospheric blurring and possibly due to difficulty knowing which shade of gray best marks the edge of the planet.

Question: Use Stellarium to find the distance to the planet when Skynet took your image, in AU. (2 points)
AU
Note: In Lab 4, you will learn how to measure distances to solar system objects directly using parallax, instead of having to look them up in Stellarium. But this will do for now.

Once the angular diameter of the planet and the distance to the planet are known, the physical diameter of the planet can be determined.
The physical diameter of the planet as a fraction of the circumference of the big circle is the same as the angular diameter of the planet as a fraction of 360°: Since circumference = 2π × radius and the radius of the big circle is the distance to the planet: Solving for the physical diameter of the planet yields:
Question: Use this equation to calculate the physical diameter of your planet in AU. You will need to convert θ to degrees first. (3 points)
AU

Show your work for both of these calculations.


Note: Since in AU, this should be a very small number.

Note: This value should be larger than the true value by the same factor that your measurement of the planet's angular diameter (θ) is larger than its true value. These factors will cancel out in Section B.4 of the procedure and consequently will not affect your measurement of the planet's mass.

Next you are going to measure the orbit of one of your planet's moons:
For each of your successful observations:
Question: Which moon did you measure? (1 point)

System Date	System Time	Julian Date (days)	Angular Separation (arcseconds)

Sign your name to attest to the fact that you collected these data yourself:

Go to this website and select "Moon". In this tutorial, you will learn how to graph your data and measure an orbit's semi-major axis in arcseconds and period in days.

Make a graph of angular separation vs. Julian date and adjust the semi-major axis (a), the orbital period (P), and the phase and tilt of the orbit until the curve best matches your data.

Note: Google the orbital period of your moon to check your result. If you are way off, try again.

Note: Jupiter, Saturn, and currently Uranus have small orbital tilts, meaning that you are viewing them close to edge on. Neptune currently has a large orbital tilt, meaning that you are viewing it close to face on. If you are way off, try again.

Save your graph as a png file.

Question: Upload your png graph. (10 points)


The orbit's semi-major axis and period are reflected in the fitted curve as follows.
The maximum angular separation between the moon and the planet is the semi-major axis of the moon's orbit.

Question: Record the semi-major axis, a, in arcseconds. (2 points)
arcseconds

The time between peaks is the time it takes for the moon to move from one side of the planet to the other. This is half of the moon's orbital period. The time between two peaks is the moon's orbital period.

Question: Record the orbital period, P, in days. (2 points)
days

Next, use your measurement of the planet's angular diameter in arcseconds and your calculation of the planet's physical diameter in AU from Section B.1 of the procedure to convert your measurement of the moon's orbital semi-major axis from arcseconds to AU.
Question: Calculate a in AU. (2 points)
AU

Show your work.
X

Note: Since in AU, this should be a very small number.

Question: Convert your measurement of the moon's orbital period from days to years. (2 points)
years

Show your work.


Note: Since in years, this should be a very small number.

By Newton's form of Kepler's third law, the mass of the planet must then be:
Question: Calculate the mass of the planet. (2 points)
solar masses

Show your work.


Question: Finally, convert the planet's mass to Earth masses: 1 solar mass = 333,000 Earth masses. (2 points)
Earth masses

Show your work.


Question: Google the true mass of your planet in Earth masses and compute your percent error. (3 points)
%

Question: Discuss significant sources of error. (3 points)

Venus's angular diameter and illuminated angular distance are defined as follows.
Use Afterglow to measure the angular diameter and illuminated angular distance of Venus (1) in your image from Lab 1, if you got one of Venus, and (2) in archival images (in Afterglow, go to "File", "Open Image(s)", "Sample Images", "Astro 101 lab", "Lab 3 - Galilean Revolution", "Venus"). Record these to the nearest 0.1 arcseconds in Data Table 2 below.

Note: For each image, select the "Histogram" window and select "MinMax". Zoom in until Venus fills your window!

Venus's phase is given by:

Compute Venus's phase for each image and record this to two decimal places in Data Table 2 below.

Image	Illuminated Angular Distance (arcseconds)	Angular Diameter (arcseconds)	Phase
1
2
3
4
5
6
7
8

Sign your name to attest to the fact that you collected these data yourself:


Go to this website and select "Venus".

Make a graph of Venus's phase vs. angular diameter. Save it as a png file.

Question: Upload your png graph. (5 points)


Question: Are your measurements more consistent with the geocentric model of the universe or the heliocentric model of the universe? Why? (3 points)


Discuss significant sources of error. (2 points)