Practical astronomy
Horizon Star Finder
Choose a place and time to see which bright stars are above the horizon, where to face, when the sky gets dark, and how a navigator can turn a star's meridian altitude into latitude.
Inputs
Which bright stars are up?
At --:-- in Sydney, 11 of the 20 brightest stars are above the horizon. Highest right now: Fomalhaut at 65.1 degrees, in the E.
Answer
11 of 20 above the horizon
Highest: Fomalhaut, 65.1 degrees altitude, 88 degrees azimuth (E).
Sun altitude: −29.3 degrees. Sky state: full astronomical night - the darkest the sky gets.
Dark window: 19:21 to 04:08
Altitude through the night
- Sirius
- Canopus
- Vega
- Acrux
Bright-star table
| Rise | Transit | Set | Status | |||
|---|---|---|---|---|---|---|
| FomalhautPiscis Austrinus, mag 1.16 | 65.1 deg | E (88 deg) | 14:41 | 22:17 | 05:48 | up |
| AltairAquila, mag 0.77 | 44 deg | NNW (335 deg) | 13:29 | 19:06 | 00:48 | up |
| AchernarEridanus, mag 0.46 | 39.1 deg | SE (139 deg) | - | 00:56 | - | circumpolar |
| AntaresScorpius, mag 1.06 | 31.8 deg | WSW (257 deg) | 08:26 | 15:46 | 23:10 | up |
| Alpha CentauriCentaurus, mag -0.27 | 26.7 deg | SSW (213 deg) | - | 13:57 | - | circumpolar |
| HadarCentaurus, mag 0.61 | 22.6 deg | SSW (211 deg) | - | 13:21 | - | circumpolar |
| AcruxCrux, mag 0.76 | 15.3 deg | SSW (201 deg) | - | 11:43 | - | circumpolar |
| MimosaCrux, mag 1.25 | 14.6 deg | SSW (206 deg) | - | 12:05 | - | circumpolar |
| DenebCygnus, mag 1.25 | 10.7 deg | N (356 deg) | 16:43 | 19:56 | 23:14 | up |
| VegaLyra, mag 0.03 | 10 deg | NNW (332 deg) | 13:59 | 17:52 | 21:49 | up |
| CanopusCarina, mag -0.74 | 3.4 deg | SSE (157 deg) | 19:22 | 05:41 | 15:56 | up |
| SpicaVirgo, mag 0.97 | −12.4 deg | WSW (247 deg) | 06:09 | 12:42 | 19:14 | down |
| ArcturusBoötes, mag -0.05 | −19.4 deg | W (280 deg) | 08:24 | 13:32 | 18:40 | down |
| RigelOrion, mag 0.13 | −21.2 deg | ESE (117 deg) | 22:09 | 04:32 | 10:56 | down |
| AldebaranTaurus, mag 0.86 | −28 deg | E (88 deg) | 22:38 | 03:54 | 09:10 | down |
| SiriusCanis Major, mag -1.46 | −29.3 deg | SE (142 deg) | 23:14 | 06:03 | 12:51 | down |
| BetelgeuseOrion, mag 0.5 | −39.2 deg | ESE (111 deg) | 23:31 | 05:13 | 10:55 | down |
| CapellaAuriga, mag 0.08 | −45.9 deg | ENE (56 deg) | 01:27 | 04:35 | 07:44 | down |
| ProcyonCanis Minor, mag 0.34 | −55.2 deg | SE (142 deg) | 01:09 | 06:57 | 12:45 | down |
| PolluxGemini, mag 1.14 | −72.5 deg | ESE (105 deg) | 02:24 | 07:03 | 11:42 | down |
Rising in the next hour: none in this 20-star reference set. Crossing the meridian now: none within 15 minutes.
Navigation worksheet
Latitude from a meridian altitude
Estimated latitude: −33.8691 degrees. Formula used: latitude = 90 - altitude + declination.
For the selected star at your current latitude, the simple transit-altitude shortcut gives 60.77 degrees before the engine's date-precession and refraction details.
Use it in code
from math import sin, cos, acos, radians, degrees
# Horizon Star Finder inputs from CalculationTime
place = "Sydney"
latitude = -33.8688
longitude = 151.2093
local_date = ""
local_time = ""
time_zone = "Australia/Sydney"
highest_star = "Fomalhaut"
highest_altitude_deg = 65.06
print(f"{place}: highest bright star is {highest_star} at {highest_altitude_deg:.2f} degrees.")The snippets print the same highest-star sentence shown in the answer card.
Print Room
One-page observing chart
CalculationTime observing sheet
Sydney star chart
Wed, 30 Sept, 20:21
At --:-- in Sydney, 11 of the 20 brightest stars are above the horizon. Highest right now: Fomalhaut at 65.1 degrees, in the E.
Dark window: 19:21 to 04:08
| Fomalhaut | 65.1 deg | E | Seen: [ ] |
| Altair | 44 deg | NNW | Seen: [ ] |
| Achernar | 39.1 deg | SE | Seen: [ ] |
| Antares | 31.8 deg | WSW | Seen: [ ] |
| Alpha Centauri | 26.7 deg | SSW | Seen: [ ] |
| Hadar | 22.6 deg | SSW | Seen: [ ] |
Method
What the finder is actually calculating
RA and Dec are the sky's address system
Right ascension is measured around the celestial equator, like longitude on the sky, and declination is measured north or south of that equator. Sirius sits near RA 6.7525 h and Dec -16.7161 degrees, while Acrux is much farther south at Dec -63.0991 degrees. The page starts from those catalog coordinates, then asks where the observer is and what the Earth has done by the chosen date and time. That last step matters: a catalog address is not a looking direction until it has been rotated through local sidereal time for one observer.
Altitude and azimuth turn that address into directions
Altitude is height above the horizon. Azimuth is the compass bearing, counted from north through east. A star at altitude 52 degrees and azimuth 205 degrees is high in the south-south-west; a star at -8 degrees is below the horizon even if it appears in the catalog. The dome uses the same numbers: centre is straight up, rim is the horizon, and the direction labels wrap around the outside. This is why changing latitude visibly moves the plotted stars even when the date and clock stay fixed.
Stars rise about 3.9 minutes earlier each night
The Earth returns to the same star-facing direction in about 23 h 56 m 4 s, not 24 hours. That four-minute daily difference accumulates. The library check for Sirius from Sydney shows its rise time moving by about 118 minutes over 30 days, which is why a winter star can become an evening star and then a dawn star across the seasons. A solar clock follows the Sun; this page is quietly showing sidereal time by letting the same stars slide earlier through the night.
Circumpolar and never-rises rules come from latitude
A star can become circumpolar when its declination is beyond 90 minus the absolute latitude on the observer's side of the sky. From London, northern stars such as Capella can stay available much longer, while far-southern stars can never rise. From Sydney the southern pole is high enough that Acrux and Alpha Centauri are familiar southern fixtures. The table keeps those three cases separate because "down right now" and "never rises here" are very different answers.
Latitude from a star is a geometry problem
At upper transit a star is crossing the meridian. If it culminates south of the zenith, latitude = 90 - altitude + declination. If it culminates north of the zenith, latitude = declination - 90 + altitude. The educational Sydney/Acrux shortcut gives 60.77 degrees and returns -33.87 degrees, but the exact engine gives about 60.63 degrees today because coordinates precess with time. The worksheet exposes both ideas so the simple navigation formula is useful without pretending it is the full astronomical model.
Brightness, twilight and honesty notes
Magnitude is logarithmic: five magnitudes is exactly a 100x brightness ratio, so one magnitude is about 2.512x. Sirius at magnitude -1.46 is therefore many times brighter than Mimosa at about 1.25, even though both are famous bright stars. Twilight matters too. Civil, nautical and astronomical twilight are based on the Sun's altitude below the horizon. Refraction, light pollution, hills and buildings can change what you actually see, so the chart is a citeable geometric planning tool rather than a promise that every star will be visible from every backyard.
Why the table and chart can disagree with your eyes
The calculation treats the horizon as a clean mathematical circle and the observer as standing at sea-level height. Real observing has tree lines, roofs, haze, moonlight, glare and imperfect vision. A star at 4 degrees altitude may be "up" in the table while still being practically hidden. That is why the printable sheet includes a seen/not-seen column: the computed sky and the observed sky are meant to be compared, not confused.
Questions people ask
Horizon Star Finder: frequently asked questions
Why are some famous bright stars missing?
This finder intentionally uses the 20-star reference set supplied for this build, not the whole sky. That keeps every row auditable and lets the chart stay readable on a phone.
Why does the result change when I move the date by one month?
Stars rise about 3 minutes 56 seconds earlier each solar day, so the same evening clock time points at a different sidereal sky a month later.
What does negative altitude mean?
Negative altitude means the star is below the mathematical horizon. It may rise later, be circumpolar, or never rise from that latitude.
Why does the sky dome put straight up in the middle?
The chart is a horizon polar plot: the centre is the zenith, the edge is the horizon, and the compass letters show direction around the rim.
Can I use the latitude tool for real navigation?
Use it as an educational reconstruction only. Real navigation also needs careful timekeeping, horizon correction, refraction, instrument error and nautical almanac procedure.
Why is the shortcut altitude slightly different from the exact engine?
The shortcut uses a star declination as if it were fixed. The engine uses coordinates precessed to the date, so Sydney Acrux is about 60.63 degrees exact rather than 60.77 degrees by the J2000 shortcut.
Certification notes
Source, method and limitation basis
Data: CalculationTime reference data v2026-09-21, coordinates verified against SIMBAD. Reviewed 2026-09-23.
Model limits
This page is for planning and education. It is not a professional navigation, aviation, surveying or legal timekeeping instrument.
Assumptions
- Star positions use the supplied CalculationTime reference data v2026-09-21 and astronomy-engine horizon calculations.
- The 20-star table is deliberately limited to the supplied bright-star dataset.
- Browser location is optional and is never required; manual latitude and longitude are enough.
- Local terrain, weather, light pollution and refraction can change practical visibility.
Cite this page
Use the canonical URL, the page title “Horizon Star Finder - CalculationTime”, and the review date 2026-09-23.