It is a testament to how people tend to hear what they want to hear. The first time was in August 2003 as people were preparing to observe the closest Mars opposition in approximately 60,000 years. A fairly innocuous statement that viewed through a telescope, Mars would look as large as the full moon with the unaided eye, turned into a widespread anticipation of how looking up, people would see Mars glowing red as large as the full Moon in the sky. And, with the idea there was this one and only opportunity to view this wondrous event. This so-called “Mars Hoax” makes a regular August reappearance for every Mars opposition, even if the opposition is taking place at a different time of year.
Oppositions of Mars take place approximately every two years, due to it having an orbital period around the Sun of about two Earth years. During most opposition years, Mars presents a brighter, and larger aspect for viewers. Less often, opposition coincides with Martian perihelion, bringing it much closer, owing to the greater eccentricity of its elliptical orbit about the Sun. During these oppositions, such as in 2003, Mars truly does provide for a magnificent sight in the sky.
Angular separation and angular diameter are important measurements in astronomy in defining the position and apparent size of celestial objects. Backyard astronomers, and learners without sophisticated equipment, can make approximate measures of either using their hands as measuring tools. An open hand with outstretched pinkie finger and thumb describes about a 20° arc; a closed fist 10°; and an extended pinkie about 1°. When applied to the full Moon, it is discovered it has an approximate angular diameter of half a pinkie, or half a degree.
Solar system scaling activities are a staple of most any astronomy professional development for educators. While there are many variations, the most basic has to do with the Earth-Moon system. With a diameter ratio between the two bodies of approximately 4:1, it is easy to model the system with four-inch, and one-inch polystyrene balls. Approximating the Earth-Moon distance as 240,000 miles, and an Earth circumference of 24,000 miles, it is easy to wrap some twine around the Earth ball ten times to find the distance to the model Moon. Due to the principal of similar triangles, the model Moon, when viewed from the position of the model Earth, will present an angular diameter of half-a-degree, or half a pinkie! A good test of the relative accuracy of a learner’s model of the system.
Once a learner creates their scale model of the Earth-Moon system, it is an easy extension to include Mars and investigate where to place the model planet so it presents the same angular diameter as the Moon. With a diameter approximately half that of Earth, a two-inch polystyrene ball can represent the Red Planet. Learners can then place the model Mars in a position where it visually appears to have the same diameter as the model Moon, discovering it is twice as far away from Earth as the Moon: about 500,000 miles! Much closer than the closest approach on July 31, 2018 when Mars comes to within 35.8 million miles. Asking the learner to find the actual position of their model Mars relative to the Earth, they discover they need to place it between a quarter and a third of a mile away at about 1,566 feet from their model Earth! They can then attempt to measure their model Mars’ angular diameter, a good task for a Galileoscope, based on knowing the angular field of view of its eyepiece.
Viewing the phenomenon of a close Mars opposition can provide a magnificent opportunity for learners new to astronomy to participate in science practices, engaging in evidence-based reasoning and modeling. Applied in a manner where learners discover for themselves the size-distance relationships, the experience may provide a powerful deterrent for the next iteration of the Mars hoax.
A version of this post first appeared in the Summer 2018 edition of Mercury Magazine, a publication of the Astronomical Society of the Pacific.