About this experiment
There is a new moon every month, so why is there not a solar eclipse every month? The answer is a tilt of just over 5 degrees: the Moon usually passes above or below the line from the Earth to the Sun. Play through any year from 2000 to 2050 and watch the eclipse seasons come round, see where the Moon shadow falls on the Earth and how deep the Moon dips into the Earth shadow. The dates and types match real eclipses.
Learning goals
- Explain how a solar eclipse and a lunar eclipse happen, using the positions of the Sun, Earth and Moon
- Explain why eclipses do not happen at every new moon and full moon
- Describe eclipse seasons and why they come about every six months
- Distinguish total, annular and partial solar eclipses, and total, partial and penumbral lunar eclipses
- Explain why a totally eclipsed Moon looks red
How to use it
- 1Play the year 2026. At which moon phase does each solar eclipse happen? And each lunar eclipse?
- 2Watch the side view at new moons that have no eclipse. Where is the Moon compared with the line to the Sun?
- 3Look at the top view during an eclipse season. Which way does the line of nodes point?
- 4Count the eclipses in several years. What is the smallest and largest number you find?
- 5Compare the annular eclipse of 17 February 2026 with the total eclipse of 12 August 2026 in the close-up. What is different?
- 6Switch on the flat orbit. How many eclipses would there be each year? Why?
The model
Assumptions
- Times and types come from Jean Meeus's algorithm in Astronomical Algorithms: mean new and full moons with the largest periodic corrections. Times are good to a few minutes; very shallow penumbral eclipses can be missed or added.
- Gamma is the distance of the shadow axis from the Earth centre in Earth radii. The solar close-up shows the shadow at greatest eclipse, not the whole track across the Earth.
- The top and side views use mean motions of the Sun, Moon and nodes, and the Moon orbit is drawn far larger than in reality so it can be seen.
- Times are in UTC with a fixed difference of 70 s between terrestrial time and UTC.
- The flat orbit is a thought experiment: the Moon keeps its real distances but orbits in the plane of the ecliptic.
Try this in class
- Q1Why can a lunar eclipse be seen from the whole night side of the Earth, but a total solar eclipse only from a narrow strip?
- Q2Why can a solar eclipse never happen at full moon?
- Q3What is the difference between an annular and a total solar eclipse? What decides which one happens?
- Q4Eclipses happen about 19 days earlier each year. Use the eclipse year to explain why.
- Q5Why does the Moon turn red during a total lunar eclipse instead of disappearing?
Frequently asked questions
How many eclipses can there be in a year?
At least four (two solar and two lunar) and at most seven. Most years have two eclipse seasons, each with one or two solar and one or two lunar eclipses. Because the eclipse year is shorter than the calendar year, a third season can start in late December.
Why is the eclipsed Moon red?
Sunlight passing through Earth's atmosphere is bent into the shadow. The atmosphere scatters blue light away, so the light that reaches the Moon is red, the light of all the sunrises and sunsets around the Earth at that moment.
Is it safe to look at an eclipse?
A lunar eclipse is completely safe to watch. Never look at the Sun directly during a solar eclipse except during the short total phase; use certified eclipse glasses or a pinhole projector instead.