Give an archive star's distance error as the error on the distance it is, not as a parallax range

formatDistance printed any error of a fifth or more as the range a parallax's error makes,
d/(1+e) to d/(1−e). A star the ETL places from the Exoplanet Archive carries the mean of sy_disterr1
and 2 instead, which the archive defines as one-sided errors on sy_dist in parsecs, and many of
those distances come from a lensing model with no parallax behind them. 129 of the 3 195 archive
stars with an error were printed as ranges the archive does not give: KMT-2016-BLG-1836L as 5.8 to
9.2 kpc, where the archive publishes 7 100 +800 −2 400 pc, and AT2021ueyL as 664 pc to 2.4 kpc
against 1 040 +740 −440.

The card now tells formatDistance when the error is on the distance (source exoplanet-archive), and
it keeps the ± at any size. Measured on the shipped catalogue: KMT-2016-BLG-1836L reads 7.1 ± 1.6 kpc,
AT2021ueyL 1.0 ± 0.6 kpc, EPIC 201170410 134 ± 43 pc, KMT-2016-BLG-0212L 6.3 ± 1.3 kpc. The ETL keeps
only the mean, so a lopsided interval such as KMT-2016-BLG-1836L's is shown symmetric; carrying both
errors would take a second column in stars-meta.bin. The doc comments on formatDistance and on
distanceError no longer say an archive distance is an inverted parallax.

Controls, each failing its named test: the error on the distance read as a parallax's (2 of 790
failed), and the card not saying it is on the distance (1 of 790).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/app/features/galaxy-system/star-readouts.spec.ts

@@ -60,6 +60,13 @@ describe('starReadouts', () => {
     expect(value(starReadouts(GAIA_STAR), 'Distance')).toBe('117 ± 12 pc');
   });
 
+  it("gives an archive star's distance error as the archive does, on the distance, not as a parallax range", () => {
+    // KMT-2016-BLG-1836L, 7 100 +800 −2 400 pc: a mean of 22.5 %.
+    const lens = { ...GAIA_STAR, x: 7100, magnitudeBand: undefined, colorIndex: null, colorSystem: undefined, distanceError: 0.2254, distanceFromGaia: false, source: 'exoplanet-archive' };
+    expect(value(starReadouts(lens), 'Distance')).toBe('7.1 ± 1.6 kpc');
+    expect(value(starReadouts({ ...lens, source: 'hyg' }), 'Distance')).toBe('5.8 kpc to 9.2 kpc');
+  });
+
   it('names the catalogue a star comes from, and whose distance it has', () => {
     expect(value(starReadouts(GAIA_STAR), 'Source')).toBe('Gaia DR3');
     expect(value(starReadouts(HYG_STAR), 'Source')).toBe('HYG');

src/app/features/galaxy-system/star-readouts.ts

@@ -43,7 +43,7 @@ export function starReadouts(star: StarRecord, surface?: StarSurface): HudReadou
   return [
     // Suppressed for the Sun rather than printed as `0.00 pc`, which is arithmetically right
     // and reads as a bug: the distance from here to here is not a measurement.
-    ...(distancePc > 0 ? [{ label: 'Distance', value: formatDistance(distancePc, star.distanceError) }] : []),
+    ...(distancePc > 0 ? [{ label: 'Distance', value: formatDistance(distancePc, star.distanceError, star.source === 'exoplanet-archive') }] : []),
     // A G magnitude and a V one are not comparable: a red dwarf is up to three brighter in G.
     { label: 'Magnitude', value: star.magnitudeBand ? `${star.magnitudeBand} ${star.magnitude.toFixed(2)}` : 'Not measured' },
     ...(star.colorIndex !== null

src/app/shared/format/quantity.spec.ts

@@ -31,6 +31,13 @@ describe('formatDistance', () => {
     expect(formatDistance(606.06, 0.45 / 1.65)).toBe('476 pc to 833 pc');
     expect(formatDistance(250, 1)).toBe('125 pc or more');
   });
+
+  it('keeps an error on the distance itself symmetric, however large', () => {
+    // AT2021ueyL: 1 040 +740 −440 pc in the archive, whose mean is 57 % of it.
+    expect(formatDistance(1040, 0.567, true)).toBe('1.0 ± 0.6 kpc');
+    expect(formatDistance(134, 0.319, true)).toBe('134 ± 43 pc');
+    expect(formatDistance(134, 0.005, true)).toBe('134 pc');
+  });
 });
 
 describe('formatAu', () => {

src/app/shared/format/quantity.ts

@@ -22,16 +22,22 @@ function parsecScale(distancePc: number): { divisor: number; digits: number; uni
  * digits the distance itself is shown to. Left off where it is 1 % or less, or would round to
  * nothing at those digits, since the figure is then already as good as it reads.
  *
- * Past a fifth, a range: the distance is the inverse of a parallax, so the parallax's symmetric
+ * Past a fifth, a range: a distance inverted from a parallax takes the parallax's symmetric error
- * error bar is a lopsided one in distance — Alnilam's 1.65 ± 0.45 mas is 476 to 833 pc, not
+ * bar as a lopsided one — Alnilam's 1.65 ± 0.45 mas is 476 to 833 pc, not 606 ± 165. Only a
- * 606 ± 165. Only a Hipparcos or archive distance gets there; Gaia's query stops at a fifth. An
+ * Hipparcos distance gets there; Gaia's query stops at a fifth. An error as large as the parallax
- * error as large as the parallax leaves no upper bound at all.
+ * leaves no upper bound at all.
+ *
+ * Not so where the error is on the distance itself, `onDistance`: the Exoplanet Archive's
+ * sy_disterr1 and 2, one-sided errors in parsecs, of which the catalogue keeps the mean. Many of
+ * those distances are no parallax at all — KMT-2016-BLG-1836L's 7.1 kpc comes from a lensing model
+ * — and read as one they gave 129 cards a range the archive does not: 5.8 to 9.2 kpc there, where
+ * it publishes 7 100 +800 −2 400 pc. They keep the ± at any size.
  */
-export function formatDistance(distancePc: number, relativeError: number | undefined): string {
+export function formatDistance(distancePc: number, relativeError: number | undefined, onDistance = false): string {
   if (relativeError === undefined || relativeError <= 0.01) {
     return formatParsecs(distancePc);
   }
-  if (relativeError < 0.2) {
+  if (relativeError < 0.2 || onDistance) {
     const { divisor, digits, unit } = parsecScale(distancePc);
     const error = ((distancePc * relativeError) / divisor).toFixed(digits);
     return Number(error) === 0 ? formatParsecs(distancePc) : `${(distancePc / divisor).toFixed(digits)} ± ${error} ${unit}`;

src/app/shared/models/star.model.ts

@@ -35,7 +35,9 @@ export interface StarRecord {
   colorSystem?: 'B-V' | 'BP-RP';
   /**
    * Relative uncertainty of the distance, σd/d: the relative error of the parallax it was
-   * inverted from, which to first order is the same. Absent where none was published.
+   * inverted from, which to first order is the same — or, for a star the Exoplanet Archive places,
+   * the mean of the two one-sided errors it gives on the distance itself, which is often no
+   * parallax's. Absent where none was published.
    */
   distanceError?: number;
   /** Whether the distance is Gaia DR3's parallax, whichever catalogue describes the star. */