Astronomy
Telescope Distance Calculator
Calculate each distance reference independently and retain its native astronomical unit so the method and scale remain visible.
Angular size (radians)-
Small-angle distance estimate (km)-
Small-angle distance (AU)-
Parallax distance (parsecs)-
Parallax distance (light-years)-
Linear Hubble-law distance (Mpc)-
Linear Hubble-law distance (million light-years)-
Decision view
Astronomical distance method map
Astronomical distance method mapSmall-angle, parallax, and linear Hubble-law distances are labeled separately because they apply to different regimes.
Exact scenario comparisonObserved angular size (arcseconds) changes while all other entered assumptions remain constant.
| Observed angular size (arcseconds) | Angular size (radians) | Small-angle distance estimate (km) | Small-angle distance (AU) | Parallax distance (parsecs) | Parallax distance (light-years) | Linear Hubble-law distance (Mpc) | Linear Hubble-law distance (million light-years) |
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How to use Telescope Distance Calculator
- Choose the method that matches the observation.
- Confirm angular and physical size describe the same dimension.
- Do not average the three outputs.
Calculator guide
Understanding Telescope Distance Calculator
Angular-size geometry, annual parallax, and linear Hubble-law distance are three different measuring regimes, not interchangeable answers.
Calculation method
How the calculation works
Provide three clearly separated distance references: small-angle geometry, stellar parallax, and a user-parameterized low-redshift linear Hubble-law estimate. Convert arcseconds to radians for the small-angle relation, invert annual parallax in arcseconds for parsecs, and divide recession velocity by the entered Hubble constant for a low-redshift Mpc reference.
Method selection
Which distance belongs to which observation?
Select by evidence, not by whichever result seems plausible.
Worked situations
Practical examples
- A parallax of 0.1 arcsecond corresponds to 10 parsecs.
- A smaller apparent angle implies a larger small-angle distance for fixed physical size.
- Peculiar velocity can dominate the Hubble estimate for nearby galaxies.
Better inputs
Useful tips
- Record angular-resolution uncertainty.
- Use catalog parallax with uncertainty when available.
- Use a cosmological calculator for meaningful redshift.
Before relying on the result
Limitations and common mistakes
- Seeing, projection, lensing, extinction, calibration, parallax bias, peculiar motion, and cosmological curvature are excluded.
- The Hubble relation is linear and user-parameterized.
- Physical size must be supplied independently.
Reference
Key terms
- Arcsecond
- One 3,600th of a degree.
- Parsec
- Distance corresponding to one arcsecond of annual parallax.
- Small-angle relation
- Physical size divided by angular size in radians.
- Hubble-law reference
- Velocity divided by an entered expansion-rate constant.
Important note
Calculated from the entered values using the displayed astronomical model. Use current ephemerides and qualified references for observation or mission decisions.
Frequently asked questions
Why are the numerical outputs so different?
They use different inputs and generally describe different distance regimes.
Can angular diameter be used directly in degrees?
Not without conversion; this page expects arcseconds.
Is one parsec one light-year?
No, one parsec is about 3.26156 light-years.
Does the Hubble result include modern cosmology?
No.