Moon Phases and Eclipses
Follow the Moon through a month of phases from the Sun, Earth and Moon's positions, and see it from the Northern or Southern Hemisphere. The half of the Moon facing the Sun is always lit; what changes is how much of it we can see. Then find out why most new and full Moons bring no eclipse, because the Moon's path is tilted, and when one does.
Preview — sign up free to change the numbers yourself
What you can adjust
- Hemisphere
- 0 – 1
- Starting day of the cycle
- 0 – 29.5 days
- Days to watch
- 1 – 60 days
- Where the paths cross
- 0 – 360 °
Where you are standing. The Moon looks turned upside down between the two hemispheres: a waxing Moon is lit on the left in the Southern Hemisphere and on the right in the Northern Hemisphere.
How many days into the 29.5-day cycle the run starts. 0 is a new Moon, about 7.4 is first quarter, about 14.8 is a full Moon and about 22.1 is last quarter.
How many days of the Moon are shown. The playback is sped up, and it slows right down for an eclipse.
The Moon's path is tilted, so it crosses Earth's path at two points. This is the angle between the Sun's direction and the line through those two points at the start. Near 0° or 180° the Sun lines up with a crossing point, and a new or full Moon there can cause an eclipse. Near 90° or 270° there are none.
About this simulation
What it shows
The Moon is followed day by day through its phases, from above and as seen from Earth in the hemisphere you pick. A strip shows a small Moon for every day, and every new and full Moon is checked for a solar or lunar eclipse.
The key idea
The half of the Moon facing the Sun is always lit; the phases come from how much of it we can see, not from Earth's shadow. Most months have no eclipse because the Moon's path is tilted 5° to Earth's, so it usually passes above or below the Sun-Earth line.
Try this
Change Where the paths cross from 351° to 90° (with 30 Days to watch): the total lunar eclipse at the full Moon disappears, because the Moon now passes about 5° from Earth's shadow.
A simple model: circular orbits at average speeds. The eclipse limits are approximate, and a real eclipse also depends on how far away the Moon is, so the kind of solar eclipse is only given as partial or central (total or annular). Rise and set times are rough and ignore season and latitude.