# J vs Moon Book

The clearest difference is review sentiment: Moon Book at 90% against J's 33%. Moon Book also leads on update cadence (every 6 months vs every 13 months) and iOS requirement (12.0 vs 15.0). On price, ads, in-app purchases and monetization model there is nothing between them.

Both are free to download. Neither carries in-app purchases, so what you see is what you pay.

J ships an update every 13 months, Moon Book every 6 months. The most recent releases landed on November 6, 2026 and November 1, 2026 respectively.

## The numbers

| Metric | J | Moon Book |
| --- | --- | --- |
| Price | Free | Free |
| Positive reviews | 33.3% of reviews | **90.0% of reviews** |
| In-app purchases | No | No |
| Ads | No | No |
| Update frequency | Every 13 months | **Every 6 months** |
| Monetization | Free | Free |
| Size | 9 MB | 22 MB |
| Requires iOS | 15.0 | **12.0** |
| Devices | iPhone, iPad, iPod | iPhone, iPad, iPod |
| Age rating | 4+ | 4+ |
| Developer | [National Research Institute for Earth Science and Disaster Prevention](https://appshunter.io/developer/427125457) | [Vixen Co.,Ltd.](https://appshunter.io/developer/536188843) |

## What reviewers say

### J

[App page](https://appshunter.io/ios/app/j-shis/id517160013)

This app provides official earthquake hazard maps for Japan, allowing users to view predictions of ground shaking intensity. It offers detailed information based on location and displays fault lines and seismic activity models.

**Highlights:** View national earthquake hazard maps, Zoom, pan, and adjust map transparency, Display probabilistic seismic hazard maps, Show surface and deep ground maps, View earthquake source fault locations, Display seismic activity models, Show location-specific earthquake probabilities

**Features:** Vision Pro support

**In-app purchases:** none


### Moon Book

[App page](https://appshunter.io/ios/app/moon-book/id906552724)

This app visualizes lunar eclipses, showing viewing times and directions from your location. Plan your observations by setting specific locations and previewing eclipse phases.

**Highlights:** Lunar eclipse viewing times, Direction of eclipse visibility, Location-specific eclipse phases, Pre-set observing locations, Overseas eclipse start times, Thumbnail images of eclipse phases

**Features:** Vision Pro support

**In-app purchases:** none


## Frequently asked questions

### Is J free?

J is free to download, with no in-app purchases.

### Is Moon Book free?

Moon Book is free to download, with no in-app purchases.

### Do J or Moon Book have ads?

Neither J nor Moon Book shows ads.

### Which is updated more often, J or Moon Book?

J ships an update every 13 months, and Moon Book every 6 months. Most recently, J was updated on November 6, 2026 and Moon Book on November 1, 2026.

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*Compared by [AppsHunter](https://appshunter.io/) from daily US App Store data. Review summaries are generated from verbatim App Store reviews.*