```typescript
/** Helper to validate finite numbers */
function assertFiniteNumber(value: number): void {
  if (typeof value !== "number" || !Number.isFinite(value)) {
    throw new Error("must be a finite number");
  }
  if (value < -273.15) {
    throw new Error("must be at or above absolute zero");
  }
}

/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  assertFiniteNumber(celsius);

  return celsius * 1.8 + 32;
}

/** The temperature in Kelvin. */
export function celsiusToKelvin(celsius: number): number {
  assertFiniteNumber(celsius);

  return celsius + 273.15;
}
```