Caitriona Jackman and the Story of the Sun-Earth-Moon Model

Caitriona Jackman is an Irish astrophysicist who is forging a stellar career as one of the top planetary scientists in the UK. Caitriona spent the majority of her career working on the Cassini-Huygens mission to study the planet Saturn and its moons. Through the story of Caitriona Jackman, we will explore the Sun-Earth-Moon model and the moons of other planets.

Dr Caitriona Jackman is from Limerick, Ireland and works as an associate professor of space physics in the University of Southampton. Jackman went to school at Crescent College Limerick where she studied physics, maths, chemistry and applied maths. In college she studied Applied Physics at the University of Limerick where she got the opportunity to work at the Mullard Space Science Laboratory for 9 months during her co-op placement. It was on her placement that she became fascinated with space physics and wanted to pursue a career in it. Her PhD gave her the proper grounding in planetary science, and a chance to work on the amazing Cassini mission to Saturn.

The Cassini probe was the first space probe to enter Saturn’s orbit where it could study the planet and its moons. Until the Cassini mission, little was known about Saturn’s largest moon Titan, save that it was a Mercury-sized world whose surface was veiled beneath a thick, nitrogen-rich atmosphere. But Cassini mapped Titan’s surface, studied its atmospheric reactions, discovered liquid seas there and even sent a probe to the moon’s surface, completely rewriting our understanding of this remarkably Earth-like world. Jackman’s team of researchers were the first people in the world to get this insight into the moon Titan. Similar studies have been done on our own moon here on earth and we can learn a lot about the physics of the solar system by studying the Sun – Earth – Moon model.

Why do the seasons change on earth?

The Sun, earth and moon have a special relationship with each other. The Earth rotates around the Sun and the moon rotates around the earth. The sun and moon have a big part to play in life on earth. The Sun provides heat and light while the moon creates the ocean’s tides. But they also control things like the seasons, day and night cycles and eclipses. We get seasons here on earth (spring, summer, autumn, winter) because of how the earth is tilted in the sky. The earth is tilted at 23.5 degrees, which means that for half a year, one half of the earth is facing the sun while the other half is not. This means that it will be hotter in one half of the earth (which we call Summer) and colder in the other half (Winter). This is why the different hemispheres have different seasons. While we are at home eating Christmas dinner, people in Australia are walking on the beach on their summer holidays.

Lunar phases

If you look up at the moon at any time of year, you’ll always see the same side of the moon. This is strange, because the moon is a spinning ball, just like the earth – shouldn’t we see it spin? Interestingly, no we shouldn’t and we never will. The earth and moon are tidally locked  which means that they both spin at the exact same rate so we will only ever see the one side. This happen to many other moons surrounding other planets and comes about from the gravitational attraction between the earth and the moon.

The moon has four phases that we can see in the night’s sky every 30 days. The phases are made by the changing position of the Sun, relative to the moon and earth. These phases are:

  1. New Moon
  2. First Quarter Moon
  3. Full Moon
  4. Last Quarter Moon

Using this simulation, we can see how the Earth, Sun and Moon move in relation to each other.


From earth, it looks like the Sun and Moon are moving across our sky, on a clear day we can even see them both in the sky at the same time. On special occasions the Sun and Moon overlap when one passes by the other. We call this an eclipse. There are two types of eclipse – solar and lunar. On earth, we get a very special relationship between the Sun and moon. The Sun is much much larger than the moon, about 400 times bigger – but, it is also about 400 times farther away from the earth, than the moon is. This means, incredibly, that when they pass each other in the sky they perfectly line up – and seem to totally cover each other. When the moon passes between the sun and the earth, (i.e. the moon covers the Sun) we call this a solar eclipse. When the earth passes between the moon and sun (i.e. the shadow of the earth covers the moon), we call this a lunar eclipse. It can be a bit confusing, this short video should help to clear all of this up!