LSTclock: sidereal time clock

LSTclock: sidereal time clock

Local sidereal time clock

10 ratings
11 reviews
Free

Details

  • Released
  • Updated

Features

LSTclock: sidereal time clock screenshot #1 for iPhone
LSTclock: sidereal time clock screenshot #2 for iPhone
LSTclock: sidereal time clock screenshot #3 for iPhone
LSTclock: sidereal time clock screenshot #4 for iPhone
LSTclock: sidereal time clock screenshot #5 for iPhone
LSTclock: sidereal time clock screenshot #6 for iPhone
LSTclock: sidereal time clock screenshot #7 for iPhone
LSTclock: sidereal time clock screenshot #8 for iPhone
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About

This utility app displays local sidereal time for registered locations on iOS, iPadOS, and watchOS. It can also calculate sidereal time and Julian Date for any date and time, and sync locations between devices.

Local sidereal time display
Julian Date calculation
Supports multiple registered locations
Location sync between iPhone and Apple Watch
Date and time specific calculations
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What's New in LSTclock

9.3

March 20, 2026

- Added a functionality to set an alarm for calendar events created from the "Choose Date/Time" sheet (iOS/iPadOS). - Modified the layout of the Main screen (watchOS). - Added an option to hide UTC/JD/MJD on the Main screen (watchOS). - UI tweaks.

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User reviews

Amazing app

Thank u for making this app, could I suggest a feature to add LST based on custom local time? I.e if local time was 3 hours in advance or less to predict future / backtrack LST?

Update: the dev just replied to me and explained the hidden Hmm feature I was not aware of: taping the play/pause button let you edit specific time! Killer feature indeed, which let me preview upcoming LST in local time!
This app is wonderful to have: works on all devices (works on Mac as well as widgets which is all I need)
Clean, modern and easy to use
It’s free!
I’m so grateful
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Exceptional and multifunctional

There is much to discover about sidereal time with this app. Take the time to read the user guide to access multiple functions.

One suggestion for the developer: a user setting to allow the app to override the lock screen timeout optionally for a constant on display setting. Thank you for bringing this very useful app to the iOS environment.
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Almost; but, not quite.

Astronomers often need to use standard and local sidereal time which is done in the app; but more importantly for record keeping, they also need Julian and Modified Julian Dates. The app does not show JD or MJD and really needs to do this to be useful and bumped from three to five stars. Having to go to settings for time (which is not self explanatory) in order to get the JD and MJD is a pain. Just display both under the LST.

Hope the developer takes this under advisement.

14 December 2024 UPDATE: Tsuyoshi Sawada quickly responded with a note through the Apple App store that he added an optional setting to switch on or off the JD/MJD in his LSTClock and I tested it this evening when making my observations. It is a perfect solution to a long term issue that I have struggled with in my work having to look things up and compute JD and MJD with online calculators. Tsuyoshi, your solution is perfect! Thank you for responding so quickly and as promised, your app is not rated ⭐️⭐️⭐️⭐️⭐️ and more. Thank you so very much!
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Almost; but, not quite.

Astronomers often need to use standard and local sidereal time which is done in the app; but more importantly for record keeping, they also need Julian and Modified Julian Dates. The app does not show JD or MJD and really needs to do this to be useful and bumped from three to five stars. Having to go to settings for time (which is not self explanatory) in order to get the JD and MJD is a pain. Just display both under the LST.

Hope the developer takes this under advisement.
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Response from developer

Thank you for the suggestion, I added an option to show JD/MJD in version 8.3.

Widget broken

Widget in iOS 18 still broken.

FAQ

What is LSTclock?

LSTclock is a free utility app for iOS, iPadOS, and watchOS that displays local sidereal time (LST) for your registered locations. It can also calculate the Julian Date for any given date and time.

What devices does LSTclock support?

LSTclock is available on iPhone, iPad, and Apple Watch. You can sync your registered locations between your iPhone and paired Apple Watch devices.

How accurate is LSTclock?

LSTclock provides calculations for sidereal time and Julian Date. While the app itself is free, user reviews indicate an average rating of 3.8 stars from 10 ratings, suggesting a generally positive but not perfect user experience.

Does LSTclock have ads?

No, LSTclock does not contain advertisements. It is a free application without any in-app purchases or ads.

When was LSTclock last updated?

The latest version of LSTclock, 9.3, was last updated on March 20, 2026. The app was initially released on April 26, 2020.

What is the age rating for LSTclock?

LSTclock has an age rating of 4+, making it suitable for a wide range of users. This rating is based on its content and is generally considered safe for children.

Can LSTclock calculate past or future sidereal times?

Yes, on iOS and iPadOS, LSTclock can calculate the local sidereal time and Julian Date for any specific date and time you input.