While analysing data taken with our Earthshine telescope, which we are running at Mauna Loa Observatory in Hawaii, we came across an image in which a meteor is crossing in front of the Moon:
Our Earthshine telescope – a mere 3 cm in diameter – is being used to monitor the amount of sunlight that bounces off the Earth and onto the Lunar disc – to appear as Earthshine or “Ashen Light” on the dark side of the Moon. The image shows the bright side of the Moon as a crescent and quite clearly the Earthshine onthe rest of the disk — with some surface features too – the lunar mare.
The exposure time is very short – a mere 9 milliseconds – to prevent overexposure of the Lunar bright side. Our telescope actually adds together a hundred such images at a time to get data we eventually analyse.
Measurements of the amount of light falling on the dark side will eventually be converted into an estimate of the albedo of the Earth: in other words, we are measuring how much light from the Sun bounces off the Earth into space – the remainder is absorbed by the Earth and warms it.
One issue with climate studies is to what extent the Earth’s albedo might change over long time scales – years to decades. On short time scales – days, weeks and months – it is affected by the development of cloud systems and changes to the vegetation and snow coverage of the continents – even by large volcanic outbursts, which can dump huge quantities of dust into the atmosphere and affect its transmission properties for months.
But back to the image – it has a bright trail crossing the lunar disk! What is it? Time to do some back of the envelope calculations, of the type we often get the students to do in SAO. The trail is about the size of the Moon’s disk (perhaps a tad shorter). The Moon is about a half degree across on the sky (varying a bit during its orbit, but that’s close enough). The object has moved 0.5 degrees in 9 milliseconds. That’s about 50 degrees per second! Such an object would only take a few seconds to cross the sky (180 degrees). Turning to an envelope back, we figure out that for a meteor to move 50 degrees per second, if it were 100 km above our heads (say), would have a speed of around 75 km/s. This seems about right. We recall that the speed of the Earth around the Sun is about 30 km/s, and meteors are born of Solar System objects in orbit around the Sun – so the impact velocity sounds about right. We decide it’s a meteor!
We hunt around with google to see if we can find an image of the Moon with a meteor – turning up nothing. Our telescope is special because it has a wide field of view, small aperture, and very fast exposure times – few observers do what we do. So perhaps this is to be expected. If any SAOers know otherwise, do let us know!
Our Earthshine experiment is blogged here . We discuss and speculate all of the science, data reduction, calibrations, telescope problems, software development, mock data modeling, new ideas, find and correct errors and problems etc etc on the blog — in other words what would normally be internal discussions in a research group are here public, something quite rare in science. The aim is to show sceince being done in practice, rather than being reported in a neat and tidy form afterwards. To the best of our knowledege, this is the only one in of its type in astronomy.
For more information, see