Frequently Asked Questions

Common questions about Byulgil and stargazing — from how the service works to tips for observing the night sky.

Q.What is Byulgil?

Byulgil is a stargazing web app that computes the night sky as it actually appears from your location — anywhere in the world — in real time. The constellations on screen are not imagined: they are the real positions of stars standing in that part of the sky right now. It also features 40+ constellations, Solar System bodies, galaxies, and reading articles.

Q.How are the star positions calculated?

Every star is stored in J2000.0 equatorial coordinates (Right Ascension and Declination), which barely change over the ages. From your observing time and location (latitude/longitude), Byulgil computes the local sidereal time and converts those coordinates into horizontal coordinates (azimuth and altitude) — the direction a star sits in the sky right now. It recomputes every second, so stars rise in the east and drift west just like the real sky.

Q.Are the displayed star positions accurate?

Byulgil computes positions from real astronomical data and coordinate transformation, so it is accurate enough for naked-eye observing and education. However, some fine corrections (atmospheric refraction, precession, and so on) are simplified, so it is not intended for celestial navigation or precise observation.

Q.How do I use AR mode?

On a mobile device, turn on AR mode and point your phone at the sky; the constellations in that direction overlay the screen. It uses the orientation sensor and compass, and corrects for magnetic declination (about 8° west in Korea) to align with true north. Desktops have no motion sensors, so AR mode is unavailable there.

Q.Is my location and personal data safe?

Your location (GPS) and orientation-sensor data are used only on your own device (in your browser) to calculate star positions — they are never transmitted to or stored on any server. Byulgil has no sign-up and does not collect identifying information such as your name or email. See the Privacy Policy for details.

Q.Do I have to allow location access?

No. If you decline location access, you can pick a major city — Seoul, Tokyo, New York, Sydney — to see that region’s sky. Choosing a southern-hemisphere city even lets you compare how the sky flips relative to the north.

Q.What is the difference between a constellation and a galaxy?

A constellation is a “picture” drawn by connecting stars, and a region of the sky; those stars are usually neighbors within our own Milky Way. A galaxy, by contrast, is a vast “island universe” of hundreds of billions of stars bound by gravity, lying millions of light-years beyond our galaxy. Byulgil introduces 40+ constellations and several notable galaxies separately.

Q.Why can’t I see some constellations from Korea?

Because Earth is round, the sky you can see depends on your latitude. From Korea (~37.5°N), far-southern constellations with very low declination — such as Crux, or Alpha and Beta Centauri — stay below the horizon and cannot be seen. Conversely, constellations near Polaris (Ursa Minor, Cassiopeia, Draco) are circumpolar and never set, so they are visible all year.

Q.What conditions are best for seeing stars?

The most important thing is a dark sky. Get away from city lights (light pollution), choose a moonless night around the new moon, and pick a clear night with low humidity. Your eyes take 20–30 minutes to adapt to the dark, so be patient, and use red light instead of bright white light to keep that adaptation. The reading section’s observing guides cover more tips.

Q.Is Byulgil free? Is there an app?

Byulgil is free to use in a web browser. There is nothing to install — just open the address and the night sky appears. On mobile, you can add it to your home screen to use it like an app.