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name: units-constants description: "Use Astropy units, quantities, equivalencies, custom units, physical types, and physical constants safely in astronomy calculations." metadata: disco-role: operating disable-model-invocation: true license: BSD 3-Clause

Units and Constants Router

Use this sub-skill when a task centers on Astropy physical units, unit-aware arrays, conversions, equivalencies, constants, or unit validation.

Load This When

• The user asks for Quantity, Unit, u.Quantity, .to(), .to_value(), .decompose(), .si, .cgs, or unit arithmetic.
• A value must carry units through NumPy operations or an astronomy formula.
• A conversion needs an equivalency: spectral wavelength/frequency/energy, Doppler velocity, temperature, parallax, brightness temperature, or dimensionless angles.
• The task defines custom units, parses unit strings, validates physical types, or needs VOUnit/FITS/CDS formatting.
• Constants such as c, G, h, M_sun, or versioned CODATA/IAU constants are involved.

Route Away When

• Coordinate frames, SkyCoord, Time, or observation locations dominate; use ../time-coordinates/SKILL.md.
• Unit-bearing table serialization is the main issue; use ../tables-io/SKILL.md.
• Cosmological units or built-in cosmology realizations dominate; use ../cosmology/SKILL.md.
• The problem is installation or optional dependency setup; use ../cli-config-data/SKILL.md or the root troubleshooting reference.

First Actions

1. Identify whether each input is a scalar, NumPy array, Quantity, Unit, string unit, or constant.
2. Convert to a Quantity early: value * u.unit or u.Quantity(value, unit).
3. Keep quantities unit-aware until an external package requires raw values.
4. Use .to(target_unit, equivalencies=...) for conversion and .to_value(...) only at boundaries.
5. If .to() fails, decide whether an equivalency is scientifically justified.
6. For constants, inspect the attached unit and whether SI/CGS ambiguity exists before using a raw .value.
7. Validate output with a dimensional check: quantity.unit.is_equivalent(...) or quantity.physical_type.

References

• references/api-reference.md lists the verified constructors and key APIs for units, quantities, equivalencies, and constants.
• references/workflows.md gives self-contained recipes for unit-aware calculations, equivalencies, custom units, NumPy interop, and constants.
• references/troubleshooting.md covers conversion errors, cgs electromagnetic ambiguity, lost units, parser strictness, and formatting pitfalls.

Safety and Validation

• Never silently strip units for convenience. If a raw number is required, state the target unit used for .to_value().
• Do not apply an equivalency unless the science context warrants it; record which equivalency was used.
• Avoid comparing raw .value across quantities with different but compatible units.
• Use constants as quantities (const.c, const.G) rather than copying numeric values into code.
• For publication or reproducibility, mention the constants version if a result depends sensitively on CODATA/IAU revisions.

Native-Backed Validation Ideas

• Convert 42 * u.km / u.s to u.m / u.s and assert 42000.
• Convert wavelength to frequency with u.spectral() and assert the output unit is equivalent to Hz.
• Use a constant in an expression and assert the resulting unit is physically expected.