```ts
/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  return _validateAndConvert(celsius, (x) => x * 1.8 + 32);
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  return _validateAndConvert(celsius, (x) => x + 273.15);
}

function _validateAndConvert<T>(value: number | null, convert: (x: number) => T): T {
  if (value === null || typeof value !== "number" || !Number.isFinite(value)) {
    throw new Error("celsius must be a finite number");
  }
  if (value < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return convert(value);
}
```