How to Use Manual Mode
  1. Enter the mean Azimuth, geographic Latitude, and Horizon Altitude for the site.
  2. Specify measurement Uncertainty (per your methods/instruments) and Alignment Tolerance for date matching.
  3. Enter the foundation Year if known; otherwise, select the relevant Century.
  4. Use negative numbers for centuries and years BCE (e.g., -1 for 1st century BCE, -27 for 27 BCE); use positive numbers for CE centuries and years (e.g., 2 for 2nd century CE, 101 for 101 CE).
  5. Click Analyze Alignment to compute and view results.

For further details about each parameter, click the info icon (i) next to each field.

  • Dating range: The available date range begins in 27 BCE (start of the Roman Empire) and extends through 476 CE (end of the Western Empire). Calculations outside this period are not supported.
  • Solar range validation: The system notifies whether the computed declination is within or outside the solar range.
  • Date matching: When using “century” mode, results refer to the midpoint year of the selected century. For more precise results, use the exact foundation year if known.
  • Input validation: Entering values that are missing or not allowed (such as latitude below 19° or above 60°) will show an error message.
  • Calendar system: All output dates are in the Julian calendar.
  • Representative date: For each alignment, the tool displays the Julian calendar date that most closely matches the computed solar declination.
Overview

ArchaeoSky is an interdisciplinary online tool for analyzing the possible astronomical orientation of Roman monuments and archaeological sites. It serves as an auxiliary resource designed to assist researchers in archaeoastronomy, archaeology, ancient history, and related fields. The tool helps users evaluate, identify, or verify potential alignments with solar phenomena—such as solstices, equinoxes, or other significant dates.

How it works

The current “Manual Mode” version calculates the geometric declination (excluding atmospheric refraction) for any azimuth, latitude, and local horizon altitude entered by the user. Users also specify a measurement uncertainty (reflecting their methods or instruments) and an alignment tolerance for date matching.

If the orientation is within the solar range, the system returns a representative date (see Manual Mode → User Guide → Show more tips). It also lists all matching Julian calendar dates—within the Roman imperial period and the selected chronology—that correspond to the calculated declination and chosen tolerance. For each matching date, the system provides the Julian Day Number (JDN).
If no astronomical alignment is possible, a relevant message is displayed.

Scientific Method

All solar positions in ArchaeoSky are computed using the Skyfield Python library (Rhodes 2019) and the high-precision NASA JPL NAIF DE431 planetary and lunar ephemerides (Folkner et al., 2014). For any user-defined azimuth, latitude, and horizon altitude, the tool calculates the declination following the method established in González-García & Lianou (2024). The Julian Day Number (JDN) for each date is computed internally by the Skyfield library. The corresponding Julian calendar date is obtained via a custom conversion function following the Meeus algorithm (Meeus 1998). All dates are referenced to the proleptic Julian calendar.

Development

ArchaeoSky aims to support rigorous research through a balance of scientific accuracy and accessibility. Future updates will include support for further analysis and advanced features.

Provided as a research prototype for academic use only. Results are preliminary and may be revised. This resource is offered as a contribution to ongoing research and welcomes feedback for further improvement.

Developed by Vasileia Lianou, 2025.

  • Folkner, W. M., Williams, J. G., Boggs, D. H., Park, R. S., & Kuchynka, P. (2014). The Planetary and Lunar Ephemerides DE430 and DE431. IPN Progress Report, 42-196.
  • González-García, A. C., & Lianou, V. (2024). The Churches of Santiago de Compostela, a diachronic analysis of their orientations. Journal of Astronomy in Culture, 2.
  • Magli, G., González-García, A. C., Belmonte, J. A., & Antonello, E. (Eds.). (2019). Archaeoastronomy in the Roman World. Springer.
  • Meeus, J. (1998). Astronomical Algorithms (2nd ed.). Willmann-Bell Inc.
  • Rhodes, B. (2019). Skyfield: High precision research-grade positions for planets and Earth satellites generator. Astrophysics Source Code Library, record ascl:1907.024.
  • Rodríguez-Antón, A., González-García, A. C., & Belmonte, J. A. (2018). An Archaeoastronomical Approach to Roman Urbanism: Orientation of Roman Settlements Across the Empire. Mediterranean Archaeology and Archaeometry, 18(4), 89–95.