Eclipses
Lunar eclipses and the red Moon: phases, colour and visibility
At a glance
A lunar eclipse occurs when a full moon passes through Earth’s shadow. It is visible from places where the Moon is above the horizon during the relevant phase. During totality it often appears reddish because sunlight passes through Earth’s atmosphere into the shadow. Apphemeride shows the contacts and which phases are visible locally.
The Moon in Earth’s shadow
Earth casts a shadow away from the Sun. A full moon lies approximately in that part of the sky, but the inclined lunar orbit normally carries it above or below the shadow.
Near an orbital node, the Moon can enter it. Inside the **umbra, Earth hides the whole Sun from a lunar viewpoint. Inside the penumbra**, only part of the solar disc is hidden. At lunar distance, the umbra is several times wider than the Moon.
Three types follow:
- Total: the whole lunar disc enters the umbra.
- Partial: only part enters the umbra.
- Penumbral: the Moon enters the penumbra without reaching the umbra, producing a subtler dimming.
There is no narrow terrestrial totality path like that of a solar eclipse. Everyone who can see the Moon shares the physical sequence, but some locations miss the beginning or end because of moonrise or moonset.
Phases and contacts
A total lunar eclipse has this sequence:
| Contact | Meaning |
|---|---|
| P1 | First contact with the penumbra |
| U1 | Entry into the umbra begins; partial phase starts |
| U2 | The whole disc is inside the umbra; totality starts |
| U3 | The disc starts leaving the umbra; totality ends |
| U4 | The Moon has completely left the umbra |
| P4 | Final exit from the penumbra |
Not every eclipse has every contact. A penumbral eclipse has no umbral entry, while a partial one has no totality. Early and late penumbral shading can be very difficult to detect by eye.
At U1, a clear dark indentation begins moving across the disc. Totality can last a little over an hour and a half in favourable geometry; the full event occupies several hours. Duration depends on the path through the shadow, distances and speeds.
**Umbral magnitude** describes penetration in units of lunar diameter. Below one the entry is partial; at or above one the Moon can fit completely within the umbra. It is not a simple percentage of surface area, and does not by itself determine what a local observer sees.
Why the Moon turns red
Earth’s umbra is not perfectly black. The atmosphere bends some sunlight into it. The long atmospheric path scatters or absorbs much of the blue component, while redder light can reach the lunar surface.
The familiar description is light from Earth’s sunrises and sunsets illuminating the Moon together. It conveys the path well, although actual colour and brightness depend on atmospheric conditions and geometry.
Dust, aerosols and clouds can make totality very dark or leave a bright copper-orange Moon. The five-step **Danjon scale** describes the observed appearance. The disc can be uneven, with areas near a shadow boundary brighter than those deeper inside.
“Blood moon” is a popular name, not a different lunar phase or a promise that every total eclipse will be vividly red. A photograph’s exposure and processing can also affect the displayed colour.
Watching and photographing safely
A lunar eclipse is safe to view directly, through binoculars or through a telescope without solar filters. You are looking at a Moon dimmer than an ordinary full moon. That statement does not apply to watching the Sun.
Check which phases occur above your horizon. The Moon may rise already eclipsed or set before the end. For a low event, a clear view towards rise or set can matter more than travelling a short distance for a slightly different clock time.
Use a stable camera support. The bright uneclipsed section and the totality disc need very different exposures, so settings may need to change throughout the event. An exposure that is too long can blur the moving Moon.
A dark totality can reveal surrounding stars that a normal full moon overwhelms. That changing sky is part of the event, even if the lunar colour is less dramatic than expected.
How Apphemeride answers this
How Apphemeride answers this
The app shows applicable P1, U1–U4 and P4 contacts, umbral and penumbral magnitudes and shadow geometry. For the selected site it indicates whether the eclipse is visible completely, in part or not at all.
Lunar data cards are free. The full version adds an eclipse simulation in real time or accelerated at 30×, 1 min/s or 2 min/s, also in 3D, and upcoming eclipse events with reminders.
Combine contacts with lunar rise, set and altitude. New York and the Grand Canyon share the physical eclipse, but not the Moon’s local position or necessarily all observable phases. Explore lunar eclipses in Apphemeride.
Frequently asked questions
What is a blood moon?
A popular name for the Moon during a total lunar eclipse. Sunlight filtered and bent through Earth’s atmosphere can make it coppery or red. The appearance varies and can be much darker than expected.
How long does a lunar eclipse last?
The complete event, including penumbral stages, can take several hours. Totality is shorter and can exceed an hour and a half in favourable cases. Partial and penumbral eclipses have different sequences and durations.
Do I need eclipse glasses?
Not to view the eclipsed Moon. Normal direct and telescopic lunar viewing is safe. Solar glasses and filters are for observing the Sun, not a Moon crossing Earth’s shadow.
Can the whole world see it?
Only places with the Moon above the horizon during each phase. Contacts are the same physical instants, but time zones change their clock notation and local rise or set can remove part of the sequence.
