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### Course: NASA>Unit 3

Lesson 2: Mars: Ancient observations

# Ancient observations

## Can you see Mars with the naked eye?

If you look at Mars in the night sky over time, you’ll notice that Mars gradually grows and shrinks in size over the course of two years.
This observed change is due to the orbital positions of Earth and Mars over time, and the shapes of Earth’s (nearly circular) and Mars’ (more elliptical) orbits. The following animations shows how the distance changes over time. Notice how they are closest anytime they form a straight line with the sun (animation not to scale):
Depending on its orbital position and Earth’s atmospheric conditions (and, today, the amount of light pollution), Mars may or may not be visible to the naked eye. As a general guide, people can see objects about 1 arcminute without telescopes or other magnifying aids.  In the 5th Century C.E., an Indian astronomical text by an unknown author estimated the angular size of Mars as 1/30 of a degree (or 2 arcminutes). This measurement was a slight overestimate compared to the actual values known today:
• At closest distance (periapsis):  25.1 arcseconds (1/143 of a degree)
• At furthest distance (apoapsis): 3.5 arcseconds (1/1028 of a degree)

## Distance from Earth to Mars

The text also calculated the distance between Earth and Mars as 10,433,000 km. This measurement was an underestimate, as we now know that the actual value ranges as follows:
• At closest distance (periapsis):  55.76 million kilometers
• At furthest distance (apoapsis):  401.3 million kilometers

## Diameter of Mars

Using the 5th Century C.E distance and angular size, the author applied basic geometry to estimate the diameter of Mars. Using these values, the diameter of Mars came out to 6,070 km. Impressively, this estimate was only 11% off of the actual values we know today: 6779 km.
Challenge Question: Next we wanted to determine the length of a year and day on Mars. What data would you need to determine this?

## Want to join the conversation?

• What is an arcminute?
• An arcminute is also known as a minute of arc. Angular distance is measured in degrees and a minute is 1/60 of a degree, just as an arc second is 1/60 of an arcminute.
• How many Marses could you fit into the margin of error for Tycho Brahe's calculation?
• Brahe's calculation was "within 4 arc minutes."
The width of mars as seen from earth varies from 3.5 to 25.1 arc seconds (we'll say 12 on avg for simplicity).
1 arc minute = 60 arc seconds
4 arcmin x 60 arcsec/arcmin x 1/12 arcsec^-1 = 20 Mars widths accuracty
• If the sun's gravity i the same, then how come the orbits of all the planets are different like Mars and Earth's?
• I'm not a scientist. I'm just a highschooler, but I'm pretty sure its because as the planets become farther away the sun's gravity has less impact on them. The size of the planets also probably as well as their own individual gravity might also have to do with it.
• Is mars going to get furthermore or closer to earth due to gravtional forces of the sun weakening?
• What is an arcminute? Why does exist in our real world?
Is it a measurement?
• An arcminute is also known as a minute of arc. Angular distance is measured in degrees and a minute is 1/60 of a degree, just as an arc second is 1/60 of an arcminute.
(1 vote)
• When is the next planned mission to Mars? Either by NASA or by ESA? Who else is able to to perform such a mission?
• There is a company that is going to be training 4 astronauts to go to mars and not return there hopes are to culturize the planet and this mission will set out in 2023.
(1 vote)
• Is there life in other planets
• We don't know. Several times scientists seem to have found alien bacteria on meteorites, but almost all of these proved to actually just have Earth bacteria on them.
(1 vote)
• I wonder, although mars has a lot of CO2 in its atmosphere, why cant plants survive on Mars on itself