```ts
/**
 * The temperature in degrees Fahrenheit or Kelvin.
 */
export function celsiusToFahrenheit(celsius: number): number;
export function celsiusToKelvin(celsius: number): number;

/** Shared validation for celsiusToFahrenheit and celsiusToKelvin. */
function validateCelsius(temp: number): void {
  if (typeof temp !== "number" || !Number.isFinite(temp)) {
    throw new Error("celsius must be a finite number");
  }
  if (temp < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }
}

export function celsiusToFahrenheit(celsius: number): number {
  validateCelsius(celsius);
  return celsius * 1.8 + 32;
}

export function celsiusToKelvin(celsius: number): number {
  validateCelsius(celsius);
  return celsius + 273.15;
}
```