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Phases of Venus

1.
Objectives

In this exercise, you will observe Venus in its orbit around the Sun and the changes in Venus’ illumination as its position relative to the Earth and the Sun changes over time. Also you will observe the difference between Venus’s Orbital (Sidereal) period and its Synodic Period.

1.
Equipment/Materials Needed

Stellarium, scientific calculator

1.
Subject Introduction

As Venus’s position changes relative to the Earth and Sun, in its orbit around the Sun, its illumination changes in what is referred to as the Phases of Venus. While most of us have seen the phases of the Moon (see the Distant Nature exercise on the Phases of the Moon), it is not possible to see the phases of Venus with out some optical aid. In fact, the first person to observe the phases of Venus was Galileo, in 1610. Take a close look at Figure 1.

Basic Venus Phases

Figure 1.

Venus orbits the Sun with a repeated period, as all planets do. The time to make one complete orbit is called the Orbital period, or the Sidereal period. For Venus to complete one complete phase cycle is different and is called the Synodic period. Unlike the Moon’s orbit around the Earth, there is quite large difference in Venus’ Synodic period versus its Orbital period.

2. Orbital period

1. The Orbital period is the time it takes an object to complete one full orbit about its companion object, relative to the stars.

2. Synodic period

2. The Synodic period is the time it takes an object to complete one full orbit about its companion object, relative to a pair of other objects. (for Venus, the two objects are the Earth and the Sun)

The other key configurations of the Sun, Earth, Venus orbital positions are shown in Figure 2. These positions were key for early astronomers to not only help solidify the heliocentric model for the solar System, but to actually allow them to calculate details about their orbit periods and shapes. Observe Figure 2.

Key Sun, Earth, Venus Positions

Figure 1.

2. Superior Conjunction

3. When an inferior planet is on the Earth-Sun line, on the far side of the Sun

2. Inferior Conjunction

4. When an inferior planet is on the Earth-Sun line, on the near side of the Sun

2. Greatest Eastern Elongation

5. The greatest angular separation of an inferior planet to the East of the Sun (seen around sunset).

2. Greatest Western Elongation

6. The greatest angular separation of an inferior planet to the West of the Sun(seen around sunrise).




Phases of Venus – Exercise Materials –


Student Name: ___________________________

1.
Exercise Activities

Step 1.

0. Start Stellarium. It should be in the default configuration you setup in the Using Stellarium exercise. You should be viewing to your South and set the program to full screen.

0. Use the Date/Time Window to set the date to 2010/1/10 and the time to 17:00:00 and leave the Date/Time Window in the upper right hand corner of the screen. Set the FOV to between 30° and 35°.

0. Move the horizon until the West compass mark is on the far right side of the screen. Keep the horizon low on the screen. You are now facing Southwest just prior to sunset during a Superior Conjunction of Venus and therefore Venus is in a Full Phase (Venus appears very close to the Sun).

0. Now use the Search Window to locate Venus.

0. Now turn off the atmosphere and the landscape.

Step 2.

0. Zoom into to a FOV of about 0.03° . You can see that, viewed from Earth, Venus is fully illuminated.

0. Use the Date/Time Window to advance the date by four months. Leave the Window up in the upper right corner.

7.
Question 1:
What is the general phase of Venus at this point? ______________

7.
Question 2:
If you noticed any other change(s) in Venus’ appearance, describe it/them.

___________________________________________

0. Advance the time by five months. you should be at 2010/10/10 .

8.
Question 3:
What is the general phase of Venus at this point? ______________

8.
Question 4:
You should now notice a significant change in Venus’ appearance, can you describe it?

_______________________________________________________

8.
Question 5:
Can you explain why this change in appearance has taken place?

_______________________________________________________

0. Continue to advance the time in one month intervals until Venus is in Full Phase.

9.
Question 6:
How many months have you advanced since you started the exercise? ______________

9.
Question 7:
What is this time in years (divide the number of months by 12 to get decimal years)?

_______________

9.
Question 8:
What is this time in days (multiply the number of decimal years by 365.25 to get decimal days)?

_______________

9.
Question 9:
Is this the Orbital Period, or the Synodic Period? _______________

Step 3.

0. Use the Date/Time Window to reset the date to 2010/1/10 and the time to 17:00:00 .

0. Using the Location Window, set the location to “Solar System Observer”. You are now looking down on the plane of the Ecliptic. From here we can observe the orbits of the inner planets

0. Use the Search Window to select the Sun.

0. Note the current position of Venus relative to the Sun

13.
Question 10:
What is the term for the currently set relationship between the Earth, Sun and Venus?

______________________________________

0. Now use the Date/Time Window to advance the date by one month.

0. Notice that as you advance the time, that all the planets move, not just Venus.

0. Now continue to advance the date in one month increments until the Sun, Earth and Venus are once again close to alignment.

16.
Question 11:
What is the term for the currently set relationship between the Earth, Sun and Venus?

______________________________________

16.
Question 12:
About how many months did you advance to get to this configuration?

____________________

0. Use the Date/Time Window to reset the date to 2010/1/10 and the time to 17:00:00 .

0. Again, note the position of Venus.

0. Advance the date in one month increments until Venus returns to a point close to the same location (relative to the Sun), but just past.

0. Now backup one month.

20.
Question 13:
About how many months did you advance to get to this configuration? (it will be between two months, so just use the half month in between, e.g. between 1 and 2 would be 1.5 months, etc.)

____________________

20.
Question 14:
How many years (in decimal) did it take for this period (divide the number of months you got by 12)?

____________________

Step 4.

0. Using the actual values for Venus:

21. Orbital = 0.62 years

21. Synodic = 583.92 days

and using Equation 1. to calculate % error.

0.
Question 15:
What is your percent error for the Orbital period?

____________________

0.
Question 16:
What is your percent error for the Synodic period?

____________________

1.
Exercise Materials

((Measured value – Actual value)/Actual value) x 100%

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