```ts
// Exported helper function to validate celsius input
export function validateCelsius(celsius: number): boolean {
  if (typeof celsius !== "number" || !Number.isFinite(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  if (celsius < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return true;
}

/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  if (!validateCelsius(celsius)) {
    throw new Error("celsius must be a finite number and at or above absolute zero");
  }
  return celsius * 1.8 + 32;
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  if (!validateCelsius(celsius)) {
    throw new Error("celsius must be a finite number and at or above absolute zero");
  }
  return celsius + 273.15;
}
```