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

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  validateCelsius(celsius);
  return celsius + 273.15;
}

function validateCelsius(celsius: number): void {
  if (typeof celsius !== "number") {
    throw new Error("celsius must be a finite number");
  }
  if (Number.isNaN(celsius)) {
    // This should never happen, but for strict type checking...
  }
}
```