Lunar Eclipses

With a total lunar eclipse coming on Jan 20/21 2019, we’ve added detailed lunar eclipse information to TPE 4.5 for iOS. Here’s what’s new.

Lunar Eclipses in the Timeline

Unlike solar eclipses, lunar eclipses are visible worldwide wherever the moon the above the horizon. They happen at the same time for all observers.

The two critical questions most photographers will want to know are “when will it start?” and “where should I look?”. 

When will it start?

The inclusion of lunar eclipse events on the timeline provides immediate answers to both. To start out, select Night Mode from the Dates and Events page, and then choose the Total Lunar Eclipse of Jan 20 (or Jan 21, depending on your timezone).

When you do so, you’ll see some new events in the timeline below the map:


The included events are as follows:

  • P1 (First contact): Penumbral eclipse start
  • U1 (Second contact): Partial eclipse start
  • U2 (Third contact): Total eclipse start
  • Time of greatest eclipse
  • U3 (Fourth contact): Total eclipse end
  • U4 (Fifth contact): Partial eclipse end
  • P4 (Sixth contact): Penumbral eclipse end

Many of you won’t be too interested in the Penumbral eclipse phases. At these times, the moon may appear slightly dimmer than usual, but it is a subtle change and one that only keen observers might notice. We only show the Penumbral events (P1 and P4) if you have “All” selected as your display option for the timeline in Night Mode.

However, if you’re planning on photographing a time lapse sequence of the eclipse, then P1 is a good time by which to get set up and ready: you’ll want to start taking exposures before the start of the partial eclipse, which is when the first significant visible changes in the moon become visible.

Totality (the time between U2 and U3) is the visual climax of the total lunar eclipse: this is the when the light level from the moon drops very significantly as it lies entirely within the “umbral cone” of Earth’s shadow. The moon’s color changes to orange-red. We used our standard “emphasis” red color to indicate this in the app — it’s hard to miss, and who doesn’t like a little eye candy for special occasions? :).

The time of greatest eclipse marks the instant when the moon lies deepest in earth’s shadow: remember, that doesn’t mean that it is necessarily a “deep” eclipse. There’s a time of greatest eclipse for only partial eclipses and also for solely penumbral eclipses. You can get a sense for how “deep” the eclipse is by inspecting the magnitude displayed at the bottom of the “Greatest eclipse” data panel.

For Jan 20/21 2019, the magnitude is 1.19. This is a unitless quantity that indicates the degree of overlap between Earth’s umbral cone and the disc of the moon. Anything over 1.0 means a total eclipse. Partial eclipses have a magnitude between 0.0 and 1.0.

For penumbral eclipses a different magnitude — the penumbral magnitude —  is displayed. This is the overlap between Earth’s penumbral cone (the ‘half shadow’ — literally halbschattenin German) and the moon’s disc. A penumbral magnitude is only displayed when it is between 0.0 and 1.0. If it were less than 0.0, then there would be no eclipse whatsoever. If it is greater than 1.0, we display the umbral magnitude instead, as there is at least a partial (and potentially a total eclipse too).

Where should I look?

Tapping on the eclipse event of interest in the timeline immediately updates the displayed time. In the screenshot below, when I tap on “Greatest” (the second red panel towards the right), the time is set to 10:12pm here in Boulder, Colorado:


From the 3D night mode display, you can immediately observe that the moon will be high in the sky to the southeast, a fact you can confirm by checking the azimuth and altitude in the legend of the altitude chart at the bottom of the screen: az: 118.53°, alt: +57.11°.

More eclipse events

You’ll now find all lunar eclipses in the events list in TPE. Previously we had events only up to 2020 listed, and did not include penumbral eclipses. Now, the app calculates all lunar and solar eclipses locally (instead of relying on a pre-canned list) and includes the event in the list.

Want to check out the solar and lunar eclipses of April 2069? No problem:


Interested in the circumstances of the partial lunar eclipse observed by Sir Ernest Shackleton and party in 1916? Go for it!



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