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What is Astrini?

Astrini is a mini-educational software designed to assist teaching of basic notions in astronomy by explaining the mechanics of:

  • seasons
  • phases of the Moon and Earth
  • eclipses of Moon and Earth

It is designed to be used in a short self-contained educational module. It may be used by the pupil on her or his own, but we recommend it for an use under the teacher’s guidance.

The key idea of Astrini is manipulation. The typical astronomy software (e.g. Stellarium) allows you to change your position in space and time, or to visualize stars below the horizon, but does not in general allow you to change any facts – it works like a virtual observatory.

Astrini, on the other hand, lets you change some facts of the model: the tilt of the Earth’s axis, and the tilt of the orbit of the Moon.

Astrini is thus not a virtual observatory. It turns Sun, Earth and Moon into toys one can play with. Why?

The actual parameters of the Sun-Earth-Moon system are a bit too complicated for one’s first steps in astronomy. By understanding how a simplified model works, one can create a confidence zone for understanding more complex facts. Curiosity for these more complex facts will be triggered by the key question, why don’t things happen as in the simplified model?

  • Why don’t we have one eclipse per month?
  • Why are day and night almost always of different length?
  • etc…

We hope you will enjoy Astrini.

What can be done in Astrini?

First choose, where to go to.

You can choose to go to the Earth, the Moon, the Sun or an outside point of view.

Direct the view

Then you can choose which object to look at.

Free camera

By clicking on Free camera you can orientate the camera as you wish by clicking and dragging in the main view. You can also move the camera by pressing the arrow keys on your keyboard. Note that when the system is in a realistic scale, it will take a considerable amount of time to travel from the Sun to the Earth, as they are very far away from each other.

Adjusting the speed

Adjust the speed of the simulation using a multiplier (x2, x20).

The Reverse button reverses the time flow, Pause it stops it. Reset brings the simulation back to the present moment and real speed.

Travel in time.

Jump forward in time, one day, one month, one season or one year.

Just below the keys for changing speed, the display keep tracks of (fictitious but realistic) time and date.

Factual changes

Important actions in Astrini are those that modify certain parameters of celestial movements.

Astrini initially presents a non-realistic astronomical layout. Thus the orbit of the Moon is not tilted relative to the plane of the Earth's orbit, and the axis of rotation of the Earth is not tilted relative to the plane of the Earth's orbit. Nevertheless you can restore the realistic parameters by activating Moon Orbit and Earth Axis. You can even exaggerate the lunar orbit bias a bit with Exaggerated Moon Orbit Tilt to make eclipses less frequent.

Astrini starts with a non-realistic spatial scale to allow you to visualize all the actors on the same scene. The System Scale button allows you to set the whole system to a realistic scale.

Display options

Toggle display options.

You can view the Shadows cast by the Earth and Moon, Orbits and Markers, as well as Stars participating in the sensation of movement in space. You an also easily spot Polaris the North Star.

Non-Blocking Interactions

We also present some educational activities to be carried out on Astrini. Whatever activity you are in, you will always be able to change your point of view, speed or object targeted in the simulation.


Do not activate any of the Factual Changes keys for the time being. Go to: Moon, Look at: Earth, and speed up time as much as you need with the Speed slider. At some point you will see the Sun pass behind the Earth, and after a while the shadow of the Moon will pass over the Earth. These are the eclipses.

Count the rotations of the Earth between two eclipses. You can count the times your favourite continent reappears. (About 14 rotations, i.e. about half a month.)

Define the Eclipses and apply your knowledge to what you observe. Activate Shadows and change viewpoint.

The good question, at this point, is: why don’t we have one eclipse of the Sun and one eclipse of the Moon each month? The answer depends from the fact that the orbit of the Moon around the Earth is not in the same plane as the orbit of the Earth around the Sun. You can see it for yourself by pressing Display: Markers and Factual changes: Moon Exaggerated Orbit Tilt. Go to any of the celestial bodies and note when you can see eclipses. When you see one, Pause, then go to other bodies or change the Speed in many ways, and comment on each perspective.

Note: in the default situation (same orbiting plane), if you go to the Moon or to the Sun and look at Earth, you will note that eclipses of the Sun occur at the terrestrial Equator only.

Days have the same length at nights only at the Equinoxes. Astrini’s default is a constant equinox! Thus, no seasons, unless you modify the Factual change: Earth tilt parameter. A good vantage point is the Sun hence Go to: Sun, then Look at: Earth.

Before the factual change, you’ll see from the Sun the same show on Earth each day, day after day. After the change, the show is different each day. One of the poles will be invisible half of the time (half year: count the revolutions of the Moon, each of which corresponds to approximately one month), while the other will be visible.

Go to: Moon and Look at: Earth. You will see the Earth spin. The Earth’s rotation was not an agreed upon fact for many centuries, because we do not have a clear feeling that the Earth spins. But if you were an astronomer living on the Moon, you would just see that the Earth spins.

Go to: Earth and Look at: Moon, and accelerate time a bit (++ a couple of times). You will see the phases of the Moon, i.e. cycles of night and day on the Moon.

Note: Astrini idealizes your position and puts you at the imaginary center of the celestial bodies in the model. Hence changes of tilt in the Earth’s axis and the rotation of the Earth do not entail different views.

It does, but as we always see the same face of the Moon, we may be inclined to think it doesn’t. Here too change of viewpoint helps realize what goes on. Go to: Sun, Look at: Moon, and you will see that te Moon does spin. From the Sun’s point of view the fact that the Moon always shows the same face to the Earth can only mean that it has to spin.

Moon and Earth have the same phase relative to the Sun, but inverted phases relative to each other’s viewpoint. Go to: Earth and Look at: Moon, and wait until New Moon appears. At this point switch viewpoints, Go to: Moon, and Look at: Earth, and you’ll see a Full Earth. If you perfom this switch at other times of the cycle, you’ll see a Waning Moon correspond to a Waxing Earth, and viceversa. And First Quarter on the Moon will correspond to Last Quarter on Earth, and viceversa.

Try out Shadows in this activity.

Observe that if you go to the Sun, it is pointless to activate Shadows. You are the source of light, therefore you cannot see the shadows you cast!


  • Idea: Roberto Casati
  • Design: Roberto Casati, Glen Lomax
  • Code: Glen Lomax

Casati, R., 2013, Dov’e il Sole di notte? Lezioni atipiche di astronomia. Milano: Raffaello Cortina.

Astrini’s first version was designed during an internship at the CogMaster (ENS-EHESS-Paris5). The current version is distributed freely under the GPL. Roberto Casati and Glen Lomax are the original developers.


Even small donations keep volunteer work afloat!


Astrini is coded in Javascript and uses BabylonJS as its 3D engine and Bootstrap for its user interface.