Local Sidereal Time: UTC: Local:
LST North: Show USNO Star Chart
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00h – 02h
Constellations:
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02h – 04h
Constellations:
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04h – 06h
Constellations:
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06h – 08h
Constellations:
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08h – 10h
Constellations:
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10h – 12h
Constellations:
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12h – 14h
Constellations:
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14h – 16h
Constellations:
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16h – 18h
Constellations:
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18h – 20h
Constellations:
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20h – 22h
Constellations:
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22h – 00h
Constellations:
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Abbreviations: Z - Zodiac Constellation | GE - Galactic Equator, Milky Way Constellation | MS - Mulitiple Star System
Notes:
Purpose, Background and Motivation: This calculator is intended to provide a light-weight non-package application that connects the user with the constellations currently overhead. Reconnection with celestial time as opposed to clock time is one of the more enjoyable effects of amateur astronomy. The digital calculator partially replicates the functionality of a physical astrolabe or planosphere.
The calculator displays the Local Sidereal Time where the meridian intersects the user's local southern horizon. The LST is fomatted as hours, minutes and seconds. A right-ascension and constellation reference table lists corresponding visible northern hemisphere constellations. Once LST is known, generally, constellations visible above the eastern horizon can be found by adding 4 hours to the LST and then consulting the corresponding table column. Constellations that are setting above the western horizon similarly are found by substracting 4 hours from the LST.
A typical use case is to link the calculator as desktop short-cut on a phone. The user can click on the link and use the LST and use the table of constellations to visualize what constellations are overhead during the day or night. With one button click, an estimated LST for tonight's nighttime observing can be found.
Another typical user case is for self-education as an aid to memorizing the order of the constellations and their main features such as bright named-stars and-or brightest Messier objects. The calculator can be used as a memory drilling device. The progression of the constellations over one entire year and for 4 ot 6 hours on a particular night can be visualized as driving a motorcycle on a large, banked circular track in a counter-clockwise direction. The constellations are visible and progress as viewed to the rider's immediate right. The entire sequence of constellations is seen across one year. A 4-6 hour subsegment of that annual progression are seen on a single night.
A final use case is in conjunction with youth education outreach programs. This LST calculator can be used to explain how the constellations progress across the night sky, how to plan for night observing, and how right ascension and solar time were used historically to tell time before the advent of mechanical and digital clocks.
Typical use of the LST Tool by this author is to view the LST on rising in the morning, at noon, and then again around 6 pm. At the evening check, a look ahead to the LST when the sun will set is consulted to roughtly plan what constellations and objects might be visible. This repetitive habit assists in memorizing the constellations and their order.
While there are many Mac Os and Android phone apps that replicate features provided here, most require large client side packages (>50mb). Motivations for writing this server side HTML version is that it has all the features that I want, is organized to my preferences, and has exclusive of a star chart, a mere 45kb footprint.
Northern Hemisphere Bias. Data in the Constellation, Bright Star and Messier Object Table and in the U.S. Cities Tables are biased towards the northern hemisphere and a 40 North latitude observing location.
Night Vision Color and Intensity. The night color HTML code used here is "deep red" - a popular choice in astronomical programs. Those programs are typically displayed on laptop or desktop screens. The "deep red" color is rendered brighter on smartphone screens. This HTML-javascript code does not adjust for screen brightness when night vision is toggled. The user should decrease their screen brightness using their smartphone or tablet device. The "deep red" color is not color blind friendly. A separate fork of this page uses the color-blind friendly code #D55E00, url: https://fisherka.csolutionshosting.net/astronote/lsttool/00000000LST_ToolCB.html.
How to Download and Fork this Page. The link to this page is: https://fisherka.csolutionshosting.net/astronote/lsttool/00000000LST_Tool.html. Create a local copy or network posted copy by downloading this page. This HTML page is self-contained except for a referenced modified USNO Bright Star Navigation Chart. Also download that modified USNO Bright Star Navigation Chart with Right Ascension and store it in the same directory as the HTML document. As a federal document, the USNO star chart is not subject to copyright.
Installing Local Copy on a Smartphone. If it is desired to run this calculator at remote sites that may not have internet access, copy the two files to a local directory on your phone. (If concerned about malware, run the document text through a AI chatbot to confirm is does not contain a virus.) Once on copied to your local phone's drive, a desktop link can be created to the local html file.
Why no GMT offset. The calculator's javascript code automatically detects the user's system time and UTC time. The GMT offset is automatically set.
Particularize to Local Observing Point. To particularize this HTML document to start with your default local latitude and longitude (expressed in decimal degrees) edit the HTML document in a text editor. Search for "40.7606" in the option value "value='40.7606'" and substitute your observing point latitude in decimal degrees. Then search for "-111.8881" and substitute your observing longitude in decimal degrees in the option value "value='-111.8881'". West longitudes are negative numbers. A table of the decimal latitude and longitude for the 100 largest U.S. cities is appended.
Why is Right Ascension also expressed as a decimal degree? The USNO Navigation Bright Star Chart is labeled in right ascension as decimal degrees.
What is the field labeled "LST North:"? The value in the "LST North:" field refers to the right ascension and decimal longitude of user's meridian line on the opposite northern horizon. When discussing the meridian and Local Sidereal Time, the right ascension at southern horizon intrinsically is referenced. The USNO Navigation Bright Star Chart only shows the quarter celestial sphere from the zenith to the southern horizon. Turn around and face north and a different set of constellations are seen. To chart those constellations, a planetarium program should be consulted. The two "LST North" field numerically estimates the right ascension where the meridian intersects the northern horizon. Again, consult a planetarium program for accurate charting. Charting of the high-circumpolar constellations are not provided here.
Clearing Browser Cache to Obtain Update to Current Version. Some browsers, particularly Google Chrome, aggressively force caching local copies of web pages. If the Chrome browser has been installed, it will control the cache of all other browsers installed on your device. This can prevent having an updated display if new versions are issued. If you suspect your browser is fixed on an outdated version, the following are some standard methods to force a hard refresh of your browser cache.
GNU Copyright © Kurt A. Fisher 2026: Some Rights Reserved. This work is free software: you can redistribute it, fork it, and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3 of the License, as amended. This work is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See the GNU General Public License for more details. The Local Sidereal Time javascript code was adapted from prior code by Victor Jiménez and Salvador Rubio, Univ. of Barcelona. The code in the current version of this document was generated with AI code development assistance.
Distribution and Mirroring Policy: You are encouraged to reproduce, host, or mirror this work on other servers, networks, or webpages, and to link directly to this page. You may do so freely and without restriction; there is no necessity to contact the author for prior permission or authorization to replicate or reference this material, provided that any derivative works retain this same copyleft declaration and license.
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Cities A – C
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Cities D – L
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Cities M – P
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Cities R – S
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Cities T – W
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End