The Radio Moon

Sue Ann Heatherly, Ron Maddalena, NRAO
Tim Spuck, OCHS

Each month, we watch the moon go through phases from a tiny curved sliver to a full round disk and back again. What would happen if we could look at the moon with a radio telescope? Would the radio moon go through phases as well? See what you can find out.

Four radio images of the moon were made in 1997 using the NRAO 140-Foot Radio Telescope in Green Bank, WV. The data were collected and processed by Dr. Ron Maddalena and Sue Ann Heatherly. The 140-Foot Telescope is the largest equatorially mounted radio telescope in the world.

Open all four radio-moon images, and arrange them so you can see all of them at the same time. Choose a palette and adjust your min/max settings for each image.

1. What are all the things you notice about these four images?

a. What similarities do you see?

b. What differences do you see? Use slice to quantify your observations. Be specific.

2. Were these images made during the same phase of the moon? Predict:

3. Go to http://www.stardate.org/nightsky/moon/ and determine if your predictions were right.

radio_moon1.fts __________________ phase _____________________________

radio_moon2.fts __________________ phase _____________________________

radio_moon3.fts _________________ phase _____________________________

radio_moon4.fts _________________ phase _____________________________

Now, what can you say about the radio moon, based on your new knowledge? Does the radio moon show phases? What new questions do you have?

Digging Deeper

In the headers of these radio images, you have information that can be used to plot the temperature of the moon for each image. Pick one of the radio moon images and select Image Info. Find BSCALE. BSCALE is a calibration number for radio images. This calibration allows you to convert brightness counts into K (or whatever unit is specified in BUNIT in the header). First , use slice to determine the brightness counts of the brightest spot on a radio moon image. Then, multiply by BSCALE to get the temperature of the moon in K. Repeat this procedure for each image.

Plot these numbers on the graph below:

Now what can you say about the radio moon?

Optical/Radio Moon Compare

In addition to the four radio moon images, open up the file "fullmoon.fts." This image was taken by Allen Galbraith on May 23, 1997 using a 35mm camera, 50 mm diameter lens and an ST-5 CCD Camera.

1. Compare fullmoon.fts image to the radio moon images. How do the radio images of the Moon differ from the optical image of the Moon?

2. Which telescope, the radio or optical, provides better resolution of the Moon? Why is this the case? (Hint: What is the difference between visible light and radio?)

Teacher notes and Extension Activities

We can see the Moon from Earth with the naked eye because the Moon reflects visible light from the Sun toward the earth. However, when you view the Moon in radio, you are not looking at reflected sunlight. Instead you are looking at reprocessed energy from the sun. Because the Moon is a "black body" it absorbs at all frequencies of radiation emitted by the Sun, and then re-emits them in a way which corresponds to the Moon's temperature. The energy picked up by the 140-Foot Radio Telescope to generate these images comes from about 1 meter below the Moon's surface.

Extension Activities:

The moon data taken at a frequency of 15 GHz. What is the corresponding wavelength of "light" in these images? (c = wavelength x frequency)

Calculate the resolution of the 140ft and the camera. Resolution of a telescope = wavelength/diameter of the lens, or mirror.

Radio Image Information:

 Image Name: radio_moon1.fts Date/Time: January 13, 1997 at 4:00 PM EST Plate Scale: 36"/pixel (along the x axis) and 42.5"/pixel (along the y axis) Image Name: radio_moon2.fts Date/Time: January 20, 1997 at 1:00 AM EST Plate Scale: 37.8"/pixel (along the x axis) and 35.8"/pixel (along the y axis) Image Name: radio_moon3.fts Date/Time: January 27, 1997 at 3:00 AM EST Plate Scale: 36.1"/pixel (along the x axis) and 31.3"/pixel (along the y axis) Image Name: radio_moon4.fts Date/Time: March 6, 1997 at 6:00 AM EST Plate Scale: 37.27"/pixel (along the x axis) and 40.9"/pixel (along the y axis) Image Name: fullmoon.fts (visible light image by Allen Galbraith) Date/Time: May 23, 1997 at 12:45 AM EST Plate Scale: 31.6"/pixel